diff --git a/src/primitives.py b/src/primitives.py
index 2caec17f547af34ff4415e1f62a9d6ed7a7c3ebd..d8361b4efb107d979b73f9bc1e462cb5536ff42a 100644
--- a/src/primitives.py
+++ b/src/primitives.py
@@ -167,8 +167,8 @@ class Expression(object):
     def is_equal(self, other):
         return NotImplemented
 
-    def get_hash(self, other):
-        raise NotImplementedError
+    def get_hash(self):
+        raise NotImplementedError("get_hash() in "+str(type(self)))