diff --git a/test/support/assertion_helpers.rb b/test/support/assertion_helpers.rb
index b4b2388b..853ccf7f 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)
-        assert #{meth}s.key?(key), "#{meth}s don't contain the given key (\#{key})"
+        assert #{meth}s.key?(key), "#{meth}s don't contain the given key (\"\#{key}\", headers: \#{#{meth}s})"
         value = #{meth}s[key]
         if value.respond_to?(:start_with?)
           assert value.start_with?(expect), "#{meth} assertion failed: \#{key}=\#{value} (expected: \#{expect}})"