diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index e6d7d76acc305..8ebfb4e393012 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,24 @@
+*   `ActiveSupport::Cache::MemCacheStore` now accepts an explicit `nil` for its `addresses` argument.
+
+    ```ruby
+    config.cache_store = :mem_cache_store, nil
+
+    # is now equivalent to
+
+    config.cache_store = :mem_cache_store
+
+    # and is also equivalent to
+
+    config.cache_store = :mem_cache_store, ENV["MEMCACHE_SERVERS"] || "localhost:11211"
+
+    # which is the fallback behavior of Dalli
+    ```
+
+    This helps those migrating from `:dalli_store`, where an explicit `nil` was permitted.
+
+    *Michael Overmeyer*
+
+
 ## Rails 6.1.1 (January 07, 2021) ##
 
 *   Change `IPAddr#to_json` to match the behavior of the json gem returning the string representation
diff --git a/activesupport/lib/active_support/cache/mem_cache_store.rb b/activesupport/lib/active_support/cache/mem_cache_store.rb
index bb6ce88206096..d783b92819183 100644
--- a/activesupport/lib/active_support/cache/mem_cache_store.rb
+++ b/activesupport/lib/active_support/cache/mem_cache_store.rb
@@ -64,7 +64,7 @@ def self.supports_cache_versioning?
       def self.build_mem_cache(*addresses) # :nodoc:
         addresses = addresses.flatten
         options = addresses.extract_options!
-        addresses = nil if addresses.empty?
+        addresses = nil if addresses.compact.empty?
         pool_options = retrieve_pool_options(options)
 
         if pool_options.empty?
diff --git a/activesupport/test/cache/stores/mem_cache_store_test.rb b/activesupport/test/cache/stores/mem_cache_store_test.rb
index b96347944f7cc..c8967f94bd425 100644
--- a/activesupport/test/cache/stores/mem_cache_store_test.rb
+++ b/activesupport/test/cache/stores/mem_cache_store_test.rb
@@ -187,6 +187,14 @@ def test_falls_back_to_localhost_if_no_address_provided_and_memcache_servers_und
     end
   end
 
+  def test_falls_back_to_localhost_if_address_provided_as_nil
+    with_memcache_servers_environment_variable(nil) do
+      cache = ActiveSupport::Cache.lookup_store(:mem_cache_store, nil)
+
+      assert_equal ["127.0.0.1:11211"], servers(cache)
+    end
+  end
+
   def test_falls_back_to_localhost_if_no_address_provided_and_memcache_servers_defined
     with_memcache_servers_environment_variable("custom_host") do
       cache = ActiveSupport::Cache.lookup_store(:mem_cache_store)