1 files changed, 5 insertions, 0 deletions
diff --git a/dice-lang/src/main/java/bjc/dicelang/ReferenceDiceExpression.java b/dice-lang/src/main/java/bjc/dicelang/ReferenceDiceExpression.java
index c1a4a79..27ec609 100644
--- a/dice-lang/src/main/java/bjc/dicelang/ReferenceDiceExpression.java
+++ b/dice-lang/src/main/java/bjc/dicelang/ReferenceDiceExpression.java
@@ -51,6 +51,11 @@ public class ReferenceDiceExpression implements IDiceExpression {
 	 */
 	@Override
 	public int roll() {
+		if (!env.containsKey(name)) {
+			throw new UnsupportedOperationException(
+					"Attempted to reference undefined variable " + name);
+		}
+		
 		return env.get(name).roll();
 	}