diff --git a/lib/slim/splat/builder.rb b/lib/slim/splat/builder.rb
index 84ecdc99..efbecb59 100644
--- a/lib/slim/splat/builder.rb
+++ b/lib/slim/splat/builder.rb
@@ -35,7 +35,11 @@ def attr(name, value)
         end
         if @attrs[name]
           if delim = @options[:merge_attrs][name]
-            @attrs[name] += delim + value.to_s
+            if @attrs[name].respond_to?(:+)
+              @attrs[name] += delim + value.to_s
+            else
+              @attrs[name] << delim + value.to_s
+            end
           else
             raise("Multiple #{name} attributes specified")
           end