diff --git a/docs/ref/models/expressions.txt b/docs/ref/models/expressions.txt
index 73c12ed5661..e59cd657bf8 100644
--- a/docs/ref/models/expressions.txt
+++ b/docs/ref/models/expressions.txt
@@ -589,7 +589,7 @@ to play nice with other query expressions::
     def get_source_expressions(self):
         return self.expressions
 
-    def set_source_expressions(expressions):
+    def set_source_expressions(self, expressions):
         self.expressions = expressions
 
 Let's see how it works::