diff --git a/test/support/assertion_helpers.rb b/test/support/assertion_helpers.rb
index 7a79c033..8fa6a572 100644
--- a/test/support/assertion_helpers.rb
+++ b/test/support/assertion_helpers.rb
@@ -12,7 +12,7 @@ module ResponseHelpers
   %w[header param].each do |meth|
     class_eval <<-DEFINE, __FILE__, __LINE__ + 1
       def verify_#{meth}(#{meth}s, key, expect)                                                                     # def verify_header(headers, key, expect)
-        assert #{meth}s.key?(key), "#{meth}s don't contain the given key (\"\#{key}\", headers: \#{#{meth}s})"      #   assert headers.key?(key), "headers ...
+        assert #{meth}s.key?(key), "#{meth}s don't contain the given key (\"\#{key.to_s}\", headers: \#{#{meth}s})" #   assert headers.key?(key), "headers ...
         value = #{meth}s[key]                                                                                       #   value = headers[key]
         if value.respond_to?(:start_with?)                                                                          #   if value.respond_to?(:start_with?)
           assert value.start_with?(expect), "#{meth} assertion failed: \#{key}=\#{value} (expected: \#{expect}})"   #     assert value.start_with?(expect), "headers assertion failed:  ...