diff --git a/com.ibm.wala.cast.python/source/com/ibm/wala/cast/python/ipa/callgraph/PythonSSAPropagationCallGraphBuilder.java b/com.ibm.wala.cast.python/source/com/ibm/wala/cast/python/ipa/callgraph/PythonSSAPropagationCallGraphBuilder.java
index 9cfc38fc..ce3929ec 100644
--- a/com.ibm.wala.cast.python/source/com/ibm/wala/cast/python/ipa/callgraph/PythonSSAPropagationCallGraphBuilder.java
+++ b/com.ibm.wala.cast.python/source/com/ibm/wala/cast/python/ipa/callgraph/PythonSSAPropagationCallGraphBuilder.java
@@ -308,7 +308,14 @@ public void visitPropertyRead(AstPropertyRead instruction) {
                         return v;
                       }
                     });
-          }
+          } else
+            throw new IllegalArgumentException(
+                "Not expecting the definition: "
+                    + def
+                    + " of the object reference of: "
+                    + instruction
+                    + " to be: "
+                    + def.getClass());
         }
 
         // check if we are reading from an module initialization script.