diff --git a/sumpy/expansion/__init__.py b/sumpy/expansion/__init__.py
index 23ba8e5b22ab0bedce3e448ac772bd830771436d..4cd9f1b63a30b1864e072ee0111fd6655868f154 100644
--- a/sumpy/expansion/__init__.py
+++ b/sumpy/expansion/__init__.py
@@ -66,8 +66,9 @@ class ExpansionBase(object):
     def __len__(self):
         return len(self.get_coefficient_identifiers())
 
-    def coefficients_from_source(self, expr, avec, bvec):
+    def coefficients_from_source(self, avec, bvec):
         """
+        :arg avec: vector from source to center.
         :arg bvec: vector from center to target. Not usually necessary,
             except for line-Taylor expansion.
         """