diff --git a/jsonpath_rw/jsonpath.py b/jsonpath_rw/jsonpath.py
index 146a960..c47653e 100644
--- a/jsonpath_rw/jsonpath.py
+++ b/jsonpath_rw/jsonpath.py
@@ -340,7 +340,7 @@ def match_recursively(datum):
                 for left_match in left_matches
                 for submatch in match_recursively(left_match)]
             
-    def is_singular():
+    def is_singular(self):
         return False
 
     def update(self, data, val):