diff --git a/src/documents/models.py b/src/documents/models.py
index 518a7b617..f051e3062 100644
--- a/src/documents/models.py
+++ b/src/documents/models.py
@@ -336,10 +336,10 @@ def many_to_dictionary(field):
             # Find delimiter
             delimiter = t.name.find('_')
 
-            if delimiter is -1:
+            if delimiter == -1:
                 delimiter = t.name.find('-')
 
-            if delimiter is -1:
+            if delimiter == -1:
                 continue
 
             key = t.name[:delimiter]