diff --git a/blockgroup.module b/blockgroup.module
index ab8a593..529e834 100644
--- a/blockgroup.module
+++ b/blockgroup.module
@@ -556,7 +556,7 @@ function blockgroup_rebuild_data() {
  *   TRUE if this region is defined by a block group, FALSE otherwise.
  */
 function blockgroup_region_is_blockgroup($region_key) {
-  return substr($region, 0, 11) === 'blockgroup_';
+  return substr($region_key, 0, 11) === 'blockgroup_';
 }