diff --git a/pandas/core/groupby.py b/pandas/core/groupby.py
index 4b85da1b7b224..774d9f651bae0 100644
--- a/pandas/core/groupby.py
+++ b/pandas/core/groupby.py
@@ -2163,7 +2163,7 @@ def _convert_grouper(axis, grouper):
             return grouper.reindex(axis).values
     elif isinstance(grouper, (list, Series, Index, np.ndarray)):
         if len(grouper) != len(axis):
-            raise AssertionError('Grouper and axis must be same length')
+            raise ValueError('Grouper and axis must be same length')
         return grouper
     else:
         return grouper
diff --git a/pandas/tests/test_groupby.py b/pandas/tests/test_groupby.py
index bf24eda60b986..ef516be28431e 100644
--- a/pandas/tests/test_groupby.py
+++ b/pandas/tests/test_groupby.py
@@ -4847,6 +4847,11 @@ def test_groupby_categorical_two_columns(self):
                          "C3":[nan,nan,nan,nan, 10,100,nan,nan, nan,nan,200,34]}, index=idx)
         tm.assert_frame_equal(res, exp)
 
+def test_convert_grouper(axis, grouper):
+    def f():
+        len(grouper) != len(axis)
+    self.assertRaisesRegexp(ValueError, 'Grouper and axis must be same length', f)
+
 
 def assert_fp_equal(a, b):
     assert (np.abs(a - b) < 1e-12).all()