diff --git a/core/src/main/java/org/nwapw/abacus/tree/NumberReducer.java b/core/src/main/java/org/nwapw/abacus/tree/NumberReducer.java
index 8584231..1cda9a5 100644
--- a/core/src/main/java/org/nwapw/abacus/tree/NumberReducer.java
+++ b/core/src/main/java/org/nwapw/abacus/tree/NumberReducer.java
@@ -28,6 +28,8 @@ public class NumberReducer implements Reducer<NumberInterface> {
     public NumberInterface reduceNode(TreeNode node, Object... children) {
         if (node instanceof NumberNode) {
             return ((NumberNode) node).getNumber();
+        } else if(node instanceof VariableNode) {
+            return abacus.numberFromString("0");
         } else if (node instanceof BinaryNode) {
             NumberInterface left = (NumberInterface) children[0];
             NumberInterface right = (NumberInterface) children[1];