diff --git a/core/src/main/java/org/nwapw/abacus/exception/PromotionException.java b/core/src/main/java/org/nwapw/abacus/exception/PromotionException.java
new file mode 100644
index 0000000..057d6c8
--- /dev/null
+++ b/core/src/main/java/org/nwapw/abacus/exception/PromotionException.java
@@ -0,0 +1,24 @@
+package org.nwapw.abacus.exception;
+
+/**
+ * Exception thrown when a promotion fails.
+ */
+public class PromotionException extends AbacusException {
+
+    /**
+     * Creates a new PromotionException with the default message
+     * and no additional information.
+     */
+    public PromotionException() {
+        this("");
+    }
+
+    /**
+     * Creates a new PromotionException with the given additional message.
+     * @param message the additional message to include with the error.
+     */
+    public PromotionException(String message) {
+        super("Failed to promote number instances.", message);
+    }
+
+}
diff --git a/core/src/main/kotlin/org/nwapw/abacus/number/PromotionManager.kt b/core/src/main/kotlin/org/nwapw/abacus/number/PromotionManager.kt
index a49a61c..e259446 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/number/PromotionManager.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/number/PromotionManager.kt
@@ -1,6 +1,7 @@
 package org.nwapw.abacus.number
 
 import org.nwapw.abacus.Abacus
+import org.nwapw.abacus.exception.PromotionException
 import org.nwapw.abacus.plugin.NumberImplementation
 import org.nwapw.abacus.plugin.PluginListener
 import org.nwapw.abacus.plugin.PluginManager
@@ -45,14 +46,14 @@ class PromotionManager(val abacus: Abacus) : PluginListener {
      * @param numbers the numbers to promote.
      * @return the resulting promotion result.
      */
-    fun promote(vararg numbers: NumberInterface): PromotionResult? {
+    fun promote(vararg numbers: NumberInterface): PromotionResult {
         val pluginManager = abacus.pluginManager
         val implementations = numbers.map { pluginManager.interfaceImplementationFor(it.javaClass) }
         val highestPriority = implementations.sortedBy { it.priority }.last()
         return PromotionResult(items = numbers.map {
             if(it.javaClass == highestPriority.implementation) it
             else computePaths[pluginManager.interfaceImplementationFor(it.javaClass) to highestPriority]
-                    ?.promote(it) ?: return null
+                    ?.promote(it) ?: throw PromotionException()
         }.toTypedArray(), promotedTo = highestPriority)
     }
 
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 7a80ca7..7eff50a 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberReducer.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberReducer.kt
@@ -36,15 +36,13 @@ 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) ?:
-                        throw EvaluationException("promotion failed.")
+                val promotionResult = promotionManager.promote(left, right)
                 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()) ?:
-                        throw EvaluationException("promotion failed.")
+                        .promote(*children.map { it as NumberInterface }.toTypedArray())
                 context.numberImplementation = promotionResult.promotedTo
                 abacus.pluginManager.functionFor(treeNode.callTo).apply(context, *promotionResult.items)
             }