diff --git a/python/pyspark/sql/dataframe.py b/python/pyspark/sql/dataframe.py
index 47d5a6a43a84dbc3379d4bcf6e3922f6f561ebc7..09647ff6d0749f658d5fe8908b49715378617d46 100644
--- a/python/pyspark/sql/dataframe.py
+++ b/python/pyspark/sql/dataframe.py
@@ -566,8 +566,7 @@ class DataFrame(object):
 
         if on is None or len(on) == 0:
             jdf = self._jdf.join(other._jdf)
-
-        if isinstance(on[0], basestring):
+        elif isinstance(on[0], basestring):
             jdf = self._jdf.join(other._jdf, self._jseq(on))
         else:
             assert isinstance(on[0], Column), "on should be Column or list of Column"