diff --git a/core/src/main/java/org/nwapw/abacus/function/DomainException.java b/core/src/main/java/org/nwapw/abacus/function/DomainException.java
index a7a449c..2d09dc3 100644
--- a/core/src/main/java/org/nwapw/abacus/function/DomainException.java
+++ b/core/src/main/java/org/nwapw/abacus/function/DomainException.java
@@ -18,7 +18,7 @@ public class DomainException extends RuntimeException {
      * Creates a new DomainException with a default message.
      */
     public DomainException(){
-        this("Domain Error");
+        this("Domain error.");
     }
 
 }
diff --git a/core/src/main/java/org/nwapw/abacus/function/EvaluationException.java b/core/src/main/java/org/nwapw/abacus/function/EvaluationException.java
index bad85b9..55db570 100644
--- a/core/src/main/java/org/nwapw/abacus/function/EvaluationException.java
+++ b/core/src/main/java/org/nwapw/abacus/function/EvaluationException.java
@@ -11,7 +11,7 @@ public class EvaluationException extends RuntimeException {
      * Creates a new EvaluationException with the default string.
      */
     public EvaluationException() {
-        super("Error evaluating expression.");
+        super("Evaluation error.");
     }
 
     /**
diff --git a/core/src/main/java/org/nwapw/abacus/number/ComputationInterruptedException.java b/core/src/main/java/org/nwapw/abacus/number/ComputationInterruptedException.java
index 2f7b3fd..e055508 100644
--- a/core/src/main/java/org/nwapw/abacus/number/ComputationInterruptedException.java
+++ b/core/src/main/java/org/nwapw/abacus/number/ComputationInterruptedException.java
@@ -10,7 +10,7 @@ public class ComputationInterruptedException extends RuntimeException {
      * Creates a new exception of this type.
      */
     public ComputationInterruptedException() {
-        super("Computation interrupted by user.");
+        super("Computation interrupted.");
     }
 
 }
diff --git a/core/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java b/core/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java
index e144bbc..edaa3a5 100755
--- a/core/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java
+++ b/core/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java
@@ -29,8 +29,7 @@ public class StandardPlugin extends Plugin {
         @Override
         public NumberInterface applyInternal(MutableEvaluationContext context, TreeNode[] params) {
             String assignTo = ((VariableNode) params[0]).getVariable();
-            NumberInterface value = params[1].reduce(context.getReducer());
-            if(value == null) throw new EvaluationException();
+            NumberInterface value = params[1].reduce(context.getInheritedReducer());
             context.setVariable(assignTo, value);
             return value;
         }
@@ -48,9 +47,7 @@ public class StandardPlugin extends Plugin {
         public NumberInterface applyInternal(MutableEvaluationContext context, TreeNode[] params) {
             String assignTo = ((VariableNode) params[0]).getVariable();
             context.setDefinition(assignTo, params[1]);
-            NumberInterface value = params[1].reduce(context.getReducer());
-            if(value == null) throw new EvaluationException();
-            return value;
+            return params[1].reduce(context.getInheritedReducer());
         }
     };
     /**
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/EvaluationResult.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/EvaluationResult.kt
index e94c9d6..a499877 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/EvaluationResult.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/EvaluationResult.kt
@@ -3,4 +3,4 @@ package org.nwapw.abacus.tree
 import org.nwapw.abacus.context.MutableEvaluationContext
 import org.nwapw.abacus.number.NumberInterface
 
-data class EvaluationResult(val value: NumberInterface?, val resultingContext: MutableEvaluationContext)
\ No newline at end of file
+data class EvaluationResult(val value: NumberInterface, val resultingContext: MutableEvaluationContext)
\ No newline at end of file
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt
index d6a7696..e4d2acb 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt
@@ -10,8 +10,8 @@ package org.nwapw.abacus.tree
  */
 class FunctionNode(function: String, children: List<TreeNode>) : CallNode(function, children) {
 
-    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
-        val children = Array<Any>(children.size, { children[it].reduce(reducer) ?: return null; })
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
+        val children = Array<Any>(children.size, { children[it].reduce(reducer) })
         return reducer.reduceNode(this, *children)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberBinaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberBinaryNode.kt
index 36f585f..3742477 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberBinaryNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberBinaryNode.kt
@@ -13,9 +13,9 @@ package org.nwapw.abacus.tree
 class NumberBinaryNode(operation: String, left: TreeNode, right: TreeNode)
     : BinaryNode(operation, left, right) {
 
-    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
-        val left = left.reduce(reducer) ?: return null
-        val right = right.reduce(reducer) ?: return null
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
+        val left = left.reduce(reducer)
+        val right = right.reduce(reducer)
         return reducer.reduceNode(this, left, right)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt
index 3507568..9278301 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt
@@ -10,7 +10,7 @@ package org.nwapw.abacus.tree
  */
 class NumberNode(val number: String) : TreeNode() {
 
-    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
         return reducer.reduceNode(this)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberReducer.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberReducer.kt
index c1a1da8..8f61923 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberReducer.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberReducer.kt
@@ -2,6 +2,7 @@ package org.nwapw.abacus.tree
 
 import org.nwapw.abacus.Abacus
 import org.nwapw.abacus.context.EvaluationContext
+import org.nwapw.abacus.function.EvaluationException
 import org.nwapw.abacus.number.NumberInterface
 
 class NumberReducer(val abacus: Abacus, context: EvaluationContext) : Reducer<NumberInterface> {
@@ -12,18 +13,19 @@ class NumberReducer(val abacus: Abacus, context: EvaluationContext) : Reducer<Nu
         this.context.reducer = this
     }
 
-    override fun reduceNode(treeNode: TreeNode, vararg children: Any): NumberInterface? {
+    override fun reduceNode(treeNode: TreeNode, vararg children: Any): NumberInterface {
         val promotionManager = abacus.promotionManager
         return when(treeNode){
             is NumberNode -> {
                 context.inheritedNumberImplementation?.instanceForString(treeNode.number)
+                        ?: throw EvaluationException()
             }
             is VariableNode -> {
                 val variable = context.getVariable(treeNode.variable)
                 if(variable != null) return variable
                 val definition = context.getDefinition(treeNode.variable)
                 if(definition != null) return definition.reduce(this)
-                null
+                throw EvaluationException()
             }
             is NumberUnaryNode -> {
                 val child = children[0] as NumberInterface
@@ -34,29 +36,29 @@ class NumberReducer(val abacus: Abacus, context: EvaluationContext) : Reducer<Nu
             is NumberBinaryNode -> {
                 val left = children[0] as NumberInterface
                 val right = children[1] as NumberInterface
-                val promotionResult = promotionManager.promote(left, right) ?: return null
+                val promotionResult = promotionManager.promote(left, right) ?: throw EvaluationException()
                 context.numberImplementation = promotionResult.promotedTo
                 abacus.pluginManager.operatorFor(treeNode.operation).apply(context, *promotionResult.items)
             }
             is FunctionNode -> {
                 val promotionResult = promotionManager
-                        .promote(*children.map { it as NumberInterface }.toTypedArray()) ?: return null
+                        .promote(*children.map { it as NumberInterface }.toTypedArray()) ?: throw EvaluationException()
                 context.numberImplementation = promotionResult.promotedTo
                 abacus.pluginManager.functionFor(treeNode.callTo).apply(context, *promotionResult.items)
             }
             is TreeValueUnaryNode -> {
                 abacus.pluginManager.treeValueOperatorFor(treeNode.operation)
-                        .apply(context, treeNode.applyTo ?: return null)
+                        .apply(context, treeNode.applyTo)
             }
             is TreeValueBinaryNode -> {
                 abacus.pluginManager.treeValueOperatorFor(treeNode.operation)
-                        .apply(context, treeNode.left ?: return null, treeNode.right ?: return null)
+                        .apply(context, treeNode.left, treeNode.right)
             }
             is TreeValueFunctionNode -> {
                 abacus.pluginManager.treeValueFunctionFor(treeNode.callTo)
                         .apply(context, *treeNode.children.toTypedArray())
             }
-            else -> null
+            else -> throw EvaluationException()
         }
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberUnaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberUnaryNode.kt
index 1dff327..01a494f 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberUnaryNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberUnaryNode.kt
@@ -11,7 +11,7 @@ package org.nwapw.abacus.tree
 class NumberUnaryNode(operation: String, child: TreeNode)
     : UnaryNode(operation, child) {
 
-    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
         val child = applyTo.reduce(reducer)
         return reducer.reduceNode(this, child)
     }
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/Reducer.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/Reducer.kt
index 412af29..45cb4dc 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/Reducer.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/Reducer.kt
@@ -14,6 +14,6 @@ interface Reducer<out T> {
      * @param treeNode the tree node to reduce.
      * @param children the list of children, of type T.
      */
-    fun reduceNode(treeNode: TreeNode, vararg children: Any): T?
+    fun reduceNode(treeNode: TreeNode, vararg children: Any): T
 
 }
\ No newline at end of file
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeNode.kt
index 2017739..a17b191 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeNode.kt
@@ -5,6 +5,6 @@ package org.nwapw.abacus.tree
  */
 abstract class TreeNode {
 
-    abstract fun <T : Any> reduce(reducer: Reducer<T>): T?
+    abstract fun <T : Any> reduce(reducer: Reducer<T>): T
 
 }
\ No newline at end of file
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueBinaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueBinaryNode.kt
index e29319b..97cd0cf 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueBinaryNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueBinaryNode.kt
@@ -14,7 +14,7 @@ package org.nwapw.abacus.tree
 class TreeValueBinaryNode(operation: String, left: TreeNode, right: TreeNode)
     : BinaryNode(operation, left, right) {
 
-    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
         return reducer.reduceNode(this)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueFunctionNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueFunctionNode.kt
index ca24d9d..18301d8 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueFunctionNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueFunctionNode.kt
@@ -9,7 +9,7 @@ package org.nwapw.abacus.tree
  */
 class TreeValueFunctionNode(name: String, children: List<TreeNode>) : CallNode(name, children) {
 
-    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
         return reducer.reduceNode(this)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueUnaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueUnaryNode.kt
index 7c3d836..dab3f52 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueUnaryNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/TreeValueUnaryNode.kt
@@ -12,8 +12,8 @@ package org.nwapw.abacus.tree
 class TreeValueUnaryNode(operation: String, child: TreeNode)
     : UnaryNode(operation, child) {
 
-    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
-        return reducer.reduceNode(this);
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
+        return reducer.reduceNode(this)
     }
 
 }
\ No newline at end of file
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt
index 93d91dc..0fdf887 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt
@@ -10,7 +10,7 @@ package org.nwapw.abacus.tree
  */
 class VariableNode(val variable: String) : TreeNode() {
 
-    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+    override fun <T : Any> reduce(reducer: Reducer<T>): T {
         return reducer.reduceNode(this)
     }
 
diff --git a/fx/src/main/java/org/nwapw/abacus/fx/AbacusController.java b/fx/src/main/java/org/nwapw/abacus/fx/AbacusController.java
index 5f23a9a..f7fb44d 100644
--- a/fx/src/main/java/org/nwapw/abacus/fx/AbacusController.java
+++ b/fx/src/main/java/org/nwapw/abacus/fx/AbacusController.java
@@ -15,6 +15,7 @@ import org.nwapw.abacus.config.Configuration;
 import org.nwapw.abacus.function.Documentation;
 import org.nwapw.abacus.function.DocumentationType;
 import org.nwapw.abacus.function.DomainException;
+import org.nwapw.abacus.function.EvaluationException;
 import org.nwapw.abacus.number.*;
 import org.nwapw.abacus.plugin.ClassFinder;
 import org.nwapw.abacus.plugin.PluginListener;
@@ -147,16 +148,11 @@ public class AbacusController implements PluginListener {
                 }
                 EvaluationResult result = abacus.evaluateTree(constructedTree);
                 NumberInterface evaluatedNumber = result.getValue();
-                if (evaluatedNumber == null) {
-                    return ERR_EVAL;
-                }
                 String resultingString = evaluatedNumber.toString();
                 historyData.add(new HistoryModel(inputField.getText(), constructedTree.toString(), resultingString));
                 abacus.applyToContext(result.getResultingContext());
                 return resultingString;
-            } catch (ComputationInterruptedException exception) {
-                return ERR_STOP;
-            } catch (DomainException exception) {
+            } catch (ComputationInterruptedException | DomainException | EvaluationException exception) {
                 return exception.getMessage();
             } catch (RuntimeException exception) {
                 exception.printStackTrace();