diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index 39ddc9a7ee22a..e0c5fa573ff69 100644
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -1040,6 +1040,10 @@ def _convert_to_indexer(self, obj, axis=0, is_setter=False):
                     level = 0
                     _, indexer = labels.reindex(objarr, level=level)
 
+                    # take all
+                    if indexer is None:
+                        indexer = np.arange(len(labels))
+
                     check = labels.levels[0].get_indexer(objarr)
                 else:
                     level = None
diff --git a/pandas/tests/test_indexing.py b/pandas/tests/test_indexing.py
index 3f6ae24756d47..fe08fb3a957c2 100644
--- a/pandas/tests/test_indexing.py
+++ b/pandas/tests/test_indexing.py
@@ -1128,6 +1128,17 @@ def test_loc_multiindex(self):
         xp = mi_int.ix[4]
         assert_frame_equal(rs,xp)
 
+        # GH6788
+        # multi-index indexer is None (meaning take all)
+        attributes = ['Attribute' + str(i) for i in range(1)]
+        attribute_values = ['Value' + str(i) for i in range(5)]
+
+        index = MultiIndex.from_product([attributes,attribute_values])
+        df = 0.1 * np.random.randn(10, 1 * 5) + 0.5
+        df = DataFrame(df, columns=index)
+        result = df[attributes]
+        assert_frame_equal(result, df)
+
     def test_series_getitem_multiindex(self):
 
         # GH 6018