diff --git a/Gemfile b/Gemfile
index 529229f..9983149 100644
--- a/Gemfile
+++ b/Gemfile
@@ -16,6 +16,11 @@ gem 'decidim', DECIDIM_VERSION
 gem 'decidim-cas_client', git: "git@gitlab.coditdev.net:decidim/decidim-cas-client.git", tag: "v0.0.19"
 gem "codit-devise-cas-authenticable", git: "git@gitlab.coditdev.net:decidim/codit-devise-cas-authenticable.git", tag: "v0.0.4"
 
+# Force gem version to fix:
+# undefined method `polymorphic?' for ActiveRecord::Reflection::PolymorphicReflection
+# See: https://github.com/activerecord-hackery/ransack/issues/1039
+gem 'ransack', '2.1.1'
+
 group :development, :test do
   gem 'byebug', platform: :mri
   gem "better_errors"
diff --git a/Gemfile.lock b/Gemfile.lock
index b212598..3b3d117 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -553,8 +553,6 @@ GEM
     pg_search (2.3.0)
       activerecord (>= 4.2)
       activesupport (>= 4.2)
-    polyamorous (2.3.0)
-      activerecord (>= 5.0)
     powerpack (0.1.2)
     premailer (1.11.1)
       addressable
@@ -604,12 +602,11 @@ GEM
       thor (>= 0.19.0, < 2.0)
     rainbow (3.0.0)
     rake (12.3.3)
-    ransack (2.3.0)
+    ransack (2.1.1)
       actionpack (>= 5.0)
       activerecord (>= 5.0)
       activesupport (>= 5.0)
       i18n
-      polyamorous (= 2.3.0)
     rb-fsevent (0.10.3)
     rb-inotify (0.10.0)
       ffi (~> 1.0)
@@ -785,6 +782,7 @@ DEPENDENCIES
   letter_opener_web (~> 1.3.0)
   listen (~> 3.1.0)
   puma (~> 3.0)
+  ransack (= 2.1.1)
   spring
   spring-watcher-listen (~> 2.0.0)
   uglifier (>= 1.3.0)