diff --git a/mozilla/webtools/bugzilla/Bugzilla/Config/Common.pm b/mozilla/webtools/bugzilla/Bugzilla/Config/Common.pm
index a609936c06a..0d6db5257e4 100644
--- a/mozilla/webtools/bugzilla/Bugzilla/Config/Common.pm
+++ b/mozilla/webtools/bugzilla/Bugzilla/Config/Common.pm
@@ -326,7 +326,7 @@ sub check_notification {
 
 sub check_timezone {
     my $tz = shift;
-    unless (tz_offset($tz)) {
+    unless (defined(tz_offset($tz))) {
         return "must be empty or a legal timezone name, such as PDT or JST";
     }
     return "";