Minor change

pystencils/cpu/cpujit.py

@@ -535,6 +535,7 @@ class ExtensionModuleCode:
         self._code_string += create_module_boilerplate_code(self._code_hash, self._function_names)
 
     def get_hash_of_code(self):
+        assert self._code_string, "The code must be generated first"
         return self._code_hash
 
     def write_to_file(self, file):