diff --git a/core/src/main/java/org/nwapw/abacus/function/DomainException.java b/core/src/main/java/org/nwapw/abacus/function/DomainException.java
new file mode 100644
index 0000000..a7a449c
--- /dev/null
+++ b/core/src/main/java/org/nwapw/abacus/function/DomainException.java
@@ -0,0 +1,24 @@
+package org.nwapw.abacus.function;
+
+/**
+ * Exception thrown if the function parameters do not match
+ * requirements.
+ */
+public class DomainException extends RuntimeException {
+
+    /**
+     * Creates a new DomainException.
+     * @param reason the reason for which the exception is thrown.
+     */
+    public DomainException(String reason) {
+        super(reason);
+    }
+
+    /**
+     * Creates a new DomainException with a default message.
+     */
+    public DomainException(){
+        this("Domain Error");
+    }
+
+}
diff --git a/core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt b/core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt
index 69f0afc..ed8bb91 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/function/applicable/Applicable.kt
@@ -1,5 +1,6 @@
 package org.nwapw.abacus.function.applicable
 
+import org.nwapw.abacus.function.DomainException
 import org.nwapw.abacus.plugin.NumberImplementation
 
 /**
@@ -25,7 +26,7 @@ interface Applicable<in T : Any, out O : Any> {
      * @param params the parameters to apply to.
      * @return the result of the application.
      */
-    fun applyInternal(implementation: NumberImplementation, params: Array<out T>): O?
+    fun applyInternal(implementation: NumberImplementation, params: Array<out T>): O
 
     /**
      * If the parameters can be used with this applicable, returns
@@ -34,8 +35,8 @@ interface Applicable<in T : Any, out O : Any> {
      * @param params the parameters to apply to.
      * @return the result of the operation, or null if parameters do not match.
      */
-    fun apply(implementation: NumberImplementation, vararg params: T): O? {
-        if (!matchesParams(implementation, params)) return null
+    fun apply(implementation: NumberImplementation, vararg params: T): O {
+        if (!matchesParams(implementation, params)) throw DomainException()
         return applyInternal(implementation, params)
     }
 
diff --git a/core/src/main/kotlin/org/nwapw/abacus/function/applicable/ReducerApplicable.kt b/core/src/main/kotlin/org/nwapw/abacus/function/applicable/ReducerApplicable.kt
index bdb3276..54340a4 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/function/applicable/ReducerApplicable.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/function/applicable/ReducerApplicable.kt
@@ -1,5 +1,6 @@
 package org.nwapw.abacus.function.applicable
 
+import org.nwapw.abacus.function.DomainException
 import org.nwapw.abacus.plugin.NumberImplementation
 import org.nwapw.abacus.tree.Reducer
 
@@ -27,7 +28,7 @@ interface ReducerApplicable<in T : Any, out O : Any, in R : Any> {
      * @param params the arguments to apply to.
      * @return the result of the application.
      */
-    fun applyWithReducerInternal(implementation: NumberImplementation, reducer: Reducer<R>, params: Array<out T>): O?
+    fun applyWithReducerInternal(implementation: NumberImplementation, reducer: Reducer<R>, params: Array<out T>): O
 
     /**
      * Applies this applicable to the given arguments, and reducer,
@@ -36,8 +37,8 @@ interface ReducerApplicable<in T : Any, out O : Any, in R : Any> {
      * @param params the arguments to apply to.
      * @return the result of the application, or null if the arguments are incompatible.
      */
-    fun applyWithReducer(implementation: NumberImplementation, reducer: Reducer<R>, vararg params: T): O? {
-        if (!matchesParams(implementation, params)) return null
+    fun applyWithReducer(implementation: NumberImplementation, reducer: Reducer<R>, vararg params: T): O {
+        if (!matchesParams(implementation, params)) throw DomainException()
         return applyWithReducerInternal(implementation, reducer, params)
     }
 
diff --git a/core/src/test/java/org/nwapw/abacus/tests/CalculationTests.java b/core/src/test/java/org/nwapw/abacus/tests/CalculationTests.java
index b210226..39cf091 100755
--- a/core/src/test/java/org/nwapw/abacus/tests/CalculationTests.java
+++ b/core/src/test/java/org/nwapw/abacus/tests/CalculationTests.java
@@ -5,6 +5,7 @@ import org.junit.BeforeClass;
 import org.junit.Test;
 import org.nwapw.abacus.Abacus;
 import org.nwapw.abacus.config.Configuration;
+import org.nwapw.abacus.function.DomainException;
 import org.nwapw.abacus.number.NumberInterface;
 import org.nwapw.abacus.plugin.StandardPlugin;
 import org.nwapw.abacus.tree.TreeNode;
@@ -28,11 +29,14 @@ public class CalculationTests {
         Assert.assertTrue(result.toString().startsWith(output));
     }
 
-    private void testEvalError(String input, String parseOutput) {
+    private void testDomainException(String input, String parseOutput) {
         TreeNode parsedTree = abacus.parseString(input);
         Assert.assertNotNull(parsedTree);
         Assert.assertEquals(parsedTree.toString(), parseOutput);
-        Assert.assertNull(abacus.evaluateTree(parsedTree));
+        try {
+            abacus.evaluateTree(parsedTree);
+            Assert.fail("Function did not throw DomainException.");
+        } catch (DomainException e){ }
     }
 
     @Test
@@ -73,7 +77,7 @@ public class CalculationTests {
 
     @Test
     public void testLn() {
-        testEvalError("ln(-1)", "ln((1)`)");
+        testDomainException("ln(-1)", "ln((1)`)");
         testOutput("ln2", "ln(2)", "0.6931471805599453094172321214581765680755");
     }
 
@@ -100,8 +104,8 @@ public class CalculationTests {
         testOutput("2^-1", "(2^(1)`)", "0.5");
         testOutput("2^50", "(2^50)", "112589990684262");
         testOutput("7^(-sqrt2*17)", "(7^((sqrt(2)*17))`)", "4.81354609155297814551845300063563");
-        testEvalError("0^0", "(0^0)");
-        testEvalError("(-13)^.9999", "((13)`^.9999)");
+        testDomainException("0^0", "(0^0)");
+        testDomainException("(-13)^.9999", "((13)`^.9999)");
     }
 
 }
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 4f62247..8b05f3f 100644
--- a/fx/src/main/java/org/nwapw/abacus/fx/AbacusController.java
+++ b/fx/src/main/java/org/nwapw/abacus/fx/AbacusController.java
@@ -14,6 +14,7 @@ import org.nwapw.abacus.Abacus;
 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.number.*;
 import org.nwapw.abacus.plugin.ClassFinder;
 import org.nwapw.abacus.plugin.PluginListener;
@@ -152,6 +153,8 @@ public class AbacusController implements PluginListener {
                 return resultingString;
             } catch (ComputationInterruptedException exception) {
                 return ERR_STOP;
+            } catch (DomainException exception) {
+                return exception.getMessage();
             } catch (RuntimeException exception) {
                 exception.printStackTrace();
                 return ERR_EXCEPTION;