diff --git a/book/_ext/pybtexapastyle/labels/apa.py b/book/_ext/pybtexapastyle/labels/apa.py
index 59176f3..3632dc6 100644
--- a/book/_ext/pybtexapastyle/labels/apa.py
+++ b/book/_ext/pybtexapastyle/labels/apa.py
@@ -56,9 +56,9 @@ class LabelStyle(BaseLabelStyle):
         #     return "{}, n.d.".format(label)
 
     def format_author_or_editor_names(self, persons):
-        if len(persons) is 1:
+        if len(persons)==1:
             return _strip_nonalnum(persons[0].last_names)
-        elif len(persons) is 2:
+        elif len(persons)==2:
             return "{} & {}".format(
                 _strip_nonalnum(persons[0].last_names),
                 _strip_nonalnum(persons[1].last_names))