diff --git a/lib/redmine_tiny_features/issue_priority_patch.rb b/lib/redmine_tiny_features/issue_priority_patch.rb
index 8b8bab8..5f48a86 100644
--- a/lib/redmine_tiny_features/issue_priority_patch.rb
+++ b/lib/redmine_tiny_features/issue_priority_patch.rb
@@ -2,7 +2,8 @@
 
 module RedmineTinyFeatures::IssuePriorityPatch
   def css_classes
-    super.to_s + " priority-#{color}"
+    default_classes = super
+    color.present? ? "#{default_classes} priority-#{color}" : default_classes
   end
 end