diff --git a/lib/guard/rails-assets.rb b/lib/guard/rails-assets.rb
index a0b2917..5714f80 100644
--- a/lib/guard/rails-assets.rb
+++ b/lib/guard/rails-assets.rb
@@ -38,7 +38,7 @@ module Guard
 
     def run_for? command
       run_on = @options[:run_on]
-      run_on = [:start, :change] if not run_on or run_on.empty? 
+      run_on = [:start, :change] if not run_on or run_on.to_s.empty?
       run_on = [run_on] unless run_on.respond_to?(:include?)
       run_on.include?(command)
     end