diff --git a/changelogs/unreleased/rails5-fix-47835.yml b/changelogs/unreleased/rails5-fix-47835.yml
new file mode 100644
index 00000000000..fe9cbf1a03a
--- /dev/null
+++ b/changelogs/unreleased/rails5-fix-47835.yml
@@ -0,0 +1,6 @@
+---
+title: Rails5 fix no implicit conversion of Hash into String. ActionController::Parameters
+  no longer returns an hash in Rails 5
+merge_request: 19792
+author: Jasper Maes
+type: fixed
diff --git a/lib/api/helpers.rb b/lib/api/helpers.rb
index 2ed331d4fd2..9c53b7c3fe7 100644
--- a/lib/api/helpers.rb
+++ b/lib/api/helpers.rb
@@ -272,7 +272,8 @@ module API
           attrs[key] = params_hash[key]
         end
       end
-      ActionController::Parameters.new(attrs).permit!
+      permitted_attrs = ActionController::Parameters.new(attrs).permit!
+      Gitlab.rails5? ? permitted_attrs.to_h : permitted_attrs
     end
 
     def filter_by_iid(items, iid)