diff --git a/pandas/tools/merge.py b/pandas/tools/merge.py
index d5bd1072f6a3e..3a99793937096 100644
--- a/pandas/tools/merge.py
+++ b/pandas/tools/merge.py
@@ -19,7 +19,7 @@
 from pandas.util.decorators import cache_readonly, Appender, Substitution
 from pandas.core.common import (PandasError, ABCSeries,
                                 is_timedelta64_dtype, is_datetime64_dtype,
-                                is_integer_dtype)
+                                is_integer_dtype, isnull)
 
 import pandas.core.common as com