diff --git a/Gemfile b/Gemfile
index c35047239..b0f0018fc 100644
--- a/Gemfile
+++ b/Gemfile
@@ -35,7 +35,8 @@ gem "simple_captcha2", "0.4.3", require: "simple_captcha"
 
 # Background processing
 
-gem "sidekiq", "5.0.4"
+gem "redis", "3.3.5" # Pinned to 3.3.x because of https://github.com/antirez/redis/issues/4272
+gem "sidekiq", "5.1.3"
 
 # Scheduled processing
 
diff --git a/Gemfile.lock b/Gemfile.lock
index 0e262532e..b5169dce3 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -583,7 +583,7 @@ GEM
     rb-inotify (0.9.10)
       ffi (>= 0.5.0, < 2)
     redcarpet (3.4.0)
-    redis (3.3.3)
+    redis (3.3.5)
     request_store (1.3.2)
     responders (2.4.0)
       actionpack (>= 4.2.0, < 5.3)
@@ -644,11 +644,11 @@ GEM
     shellany (0.0.1)
     shoulda-matchers (3.1.2)
       activesupport (>= 4.0.0)
-    sidekiq (5.0.4)
+    sidekiq (5.1.3)
       concurrent-ruby (~> 1.0)
       connection_pool (~> 2.2, >= 2.2.0)
       rack-protection (>= 1.5.0)
-      redis (~> 3.3, >= 3.3.3)
+      redis (>= 3.3.5, < 5)
     sidekiq-cron (0.6.3)
       rufus-scheduler (>= 3.3.0)
       sidekiq (>= 4.2.1)
@@ -874,6 +874,7 @@ DEPENDENCIES
   rb-fsevent (= 0.10.2)
   rb-inotify (= 0.9.10)
   redcarpet (= 3.4.0)
+  redis (= 3.3.5)
   responders (= 2.4.0)
   rspec-json_expectations (~> 2.1)
   rspec-rails (= 3.6.1)
@@ -883,7 +884,7 @@ DEPENDENCIES
   sass-rails (= 5.0.6)
   secure_headers (= 3.7.1)
   shoulda-matchers (= 3.1.2)
-  sidekiq (= 5.0.4)
+  sidekiq (= 5.1.3)
   sidekiq-cron (= 0.6.3)
   simple_captcha2 (= 0.4.3)
   simplecov (= 0.14.1)