diff --git a/mailsystem.theme.inc b/mailsystem.theme.inc
index 0e5c0dc..d789fa5 100644
--- a/mailsystem.theme.inc
+++ b/mailsystem.theme.inc
@@ -83,6 +83,9 @@ function mailsystem_theme_theme_registry_alter(&$theme_registry) {
         // Swap back to the original theme
         mailsystem_theme_swap_theme($old_theme);
       }
+      
+      // Clear theme registry cache.
+      cache_clear_all('theme_registry', 'cache', TRUE);
     }
   }
   $recursion_prevention = FALSE;