diff --git a/lib/roda/proxy.rb b/lib/roda/proxy.rb
index 4fc9800..a39def9 100644
--- a/lib/roda/proxy.rb
+++ b/lib/roda/proxy.rb
@@ -138,7 +138,7 @@ class Roda
         def _proxy_headers
           env_host = env['HTTP_HOST'].to_s
           env_new = env
-            .select { |k, _v| k.start_with? 'HTTP_' }
+            .select { |k, _v| k.to_s.start_with? 'HTTP_' }
             .reject { |k, _v| k == 'HTTP_HOST' }
             .transform_keys do |k| 
               k.sub(/^HTTP_/, '')