diff --git a/dfply/group.py b/dfply/group.py
index 1dd74f5..ae3f525 100644
--- a/dfply/group.py
+++ b/dfply/group.py
@@ -4,7 +4,7 @@
 @pipe
 @symbolic_evaluation(eval_as_label=True)
 def group_by(df, *args):
-    df._grouped_by = args
+    df._grouped_by = list(args)
     return df
 
 
diff --git a/test/test_group.py b/test/test_group.py
index 4f0c456..f3ecc32 100644
--- a/test/test_group.py
+++ b/test/test_group.py
@@ -10,4 +10,4 @@
 def test_group_attributes():
     d = diamonds >> group_by('cut')
     assert hasattr(d, '_grouped_by')
-    assert d._grouped_by == ('cut',)
+    assert d._grouped_by == ['cut',]