diff --git a/lib/rabl-rails/library.rb b/lib/rabl-rails/library.rb
index 861fe96..a421416 100644
--- a/lib/rabl-rails/library.rb
+++ b/lib/rabl-rails/library.rb
@@ -15,7 +15,7 @@ module RablRails
       compiled_template = compile_template_from_source(source, path)
 
       format = context.params[:format] || 'json'
-      Renderers.const_get(format.upcase!).new(context, locals).render(compiled_template)
+      Renderers.const_get(format.to_s.upcase!).new(context, locals).render(compiled_template)
     end
 
     def compile_template_from_source(source, path = nil)