diff --git a/plugins/filter/lists_mergeby.py b/plugins/filter/lists_mergeby.py
index d66933de..eddda08f 100644
--- a/plugins/filter/lists_mergeby.py
+++ b/plugins/filter/lists_mergeby.py
@@ -99,7 +99,6 @@
 '''
 
 from ansible.errors import AnsibleFilterError
-from ansible.module_utils.six import string_types
 from ansible.module_utils.common._collections_compat import Mapping, Sequence
 from ansible.utils.vars import merge_hash
 
@@ -170,7 +169,7 @@ def lists_mergeby(*terms, **kwargs):
 
     index = terms[-1]
 
-    if not isinstance(index, string_types):
+    if not isinstance(index, str):
         msg = ("First argument after the lists for lists_mergeby must be string. "
                "%s is %s")
         raise AnsibleFilterError(msg % (index, type(index)))