diff --git a/Changes.md b/Changes.md
index f121156f..5f6ded7e 100644
--- a/Changes.md
+++ b/Changes.md
@@ -1,3 +1,10 @@
+HEAD
+-----------
+
+- Raise error if application tries to configure Redis after the pool has been
+  established.  Sidekiq's redis connection cannot be dynamically changed
+  at runtime. [#1309]
+
 2.17.0
 -----------
 
diff --git a/lib/sidekiq.rb b/lib/sidekiq.rb
index 8e8cec54..b7204de8 100644
--- a/lib/sidekiq.rb
+++ b/lib/sidekiq.rb
@@ -68,6 +68,7 @@ module Sidekiq
   end
 
   def self.redis=(hash)
+    raise "Sidekiq's redis pool has already been established, cannot configure" if @redis && !$TESTING
     return @redis = hash if hash.is_a?(ConnectionPool)
 
     if hash.is_a?(Hash)