diff --git a/ink-engine-runtime/Story.cs b/ink-engine-runtime/Story.cs
index cd9a5cef..5ac9d30f 100644
--- a/ink-engine-runtime/Story.cs
+++ b/ink-engine-runtime/Story.cs
@@ -1838,7 +1838,7 @@ public void CallExternalFunction(string funcName, int numberOfArguments)
             var arguments = new List<object>();
             for (int i = 0; i < numberOfArguments; ++i) {
                 var poppedObj = state.PopEvaluationStack () as Value;
-                var valueObj = poppedObj.valueObject;
+                var valueObj = poppedObj?.valueObject;
                 arguments.Add (valueObj);
             }