1 files changed, 9 insertions, 2 deletions
diff --git a/actionpack/lib/action_controller/metal/strong_parameters.rb b/actionpack/lib/action_controller/metal/strong_parameters.rb
index 66403d533c..b4948d99a8 100644
--- a/actionpack/lib/action_controller/metal/strong_parameters.rb
+++ b/actionpack/lib/action_controller/metal/strong_parameters.rb
@@ -17,7 +17,7 @@ module ActionController
 
     def initialize(param) # :nodoc:
       @param = param
-      super("param not found: #{param}")
+      super("param is missing or the value is empty: #{param}")
     end
   end
 
@@ -284,7 +284,14 @@ module ActionController
     #   params.fetch(:none, 'Francesco')    # => "Francesco"
     #   params.fetch(:none) { 'Francesco' } # => "Francesco"
     def fetch(key, *args)
-      convert_hashes_to_parameters(key, super)
+      value = super
+      # Don't rely on +convert_hashes_to_parameters+
+      # so as to not mutate via a +fetch+
+      if value.is_a?(Hash)
+        value = self.class.new(value)
+        value.permit! if permitted?
+      end
+      value
     rescue KeyError
       raise ActionController::ParameterMissing.new(key)
     end