diff --git a/lib/sprockets/processing.rb b/lib/sprockets/processing.rb
index e8a00abdd..178167e17 100644
--- a/lib/sprockets/processing.rb
+++ b/lib/sprockets/processing.rb
@@ -268,11 +268,11 @@ def js_compressor=(compressor)
 
     private
       def add_engine_to_trail(ext, klass)
-        @trail.append_extension(ext)
+        @trail.append_extension(ext.to_s)
 
         if klass.respond_to?(:default_mime_type) && klass.default_mime_type
           if format_ext = extension_for_mime_type(klass.default_mime_type)
-            @trail.alias_extension(ext, format_ext)
+            @trail.alias_extension(ext.to_s, format_ext)
           end
         end
       end