diff --git a/lib/stripe/util.rb b/lib/stripe/util.rb
index 2319869b..5149c8f8 100644
--- a/lib/stripe/util.rb
+++ b/lib/stripe/util.rb
@@ -124,7 +124,7 @@ module Stripe
       when NilClass
         {}
       when String
-        {api_key: opts}
+        {:api_key => opts}
       when Hash
         opts.clone
       else