diff --git a/lib/refinements/hashes.rb b/lib/refinements/hashes.rb
index 52c3bdd..dcf51f4 100644
--- a/lib/refinements/hashes.rb
+++ b/lib/refinements/hashes.rb
@@ -55,7 +55,7 @@ def fetch_value(key, *default, &) = fetch(key, *default, &) || default.first
 
       def flatten_keys prefix: nil, delimiter: "_"
         reduce({}) do |accumulator, (key, value)|
-          flat_key = prefix ? "#{prefix}#{delimiter}#{key}".to_sym : key
+          flat_key = prefix ? :"#{prefix}#{delimiter}#{key}" : key
 
           next accumulator.merge flat_key => value unless value in Hash