diff --git a/lib/active_model/serializer/config.rb b/lib/active_model/serializer/config.rb
index 3b5d9a44..1493fd5a 100644
--- a/lib/active_model/serializer/config.rb
+++ b/lib/active_model/serializer/config.rb
@@ -1,35 +1,48 @@
+require 'thread'
+
 module ActiveModel
   class Serializer
     class Config
       def initialize
         @data = {}
+        @mutex = Mutex.new
       end
 
       def [](key)
-        @data[key.to_s]
+        @mutex.synchronize do
+          @data[key.to_s]
+        end
       end
 
       def []=(key, value)
-        @data[key.to_s] = value
+        @mutex.synchronize do
+          @data[key.to_s] = value
+        end
       end
 
       def each(&block)
-        @data.each(&block)
+        @mutex.synchronize do
+          @data.each(&block)
+        end
       end
 
       def clear
-        @data.clear
+        @mutex.synchronize do
+          @data.clear
+        end
       end
 
       def method_missing(name, *args)
-        name = name.to_s
-        return @data[name] if @data.include?(name)
-        match = name.match(/\A(.*?)([?=]?)\Z/)
-        case match[2]
-        when "="
-          @data[match[1]] = args.first
-        when "?"
-          !!@data[match[1]]
+        @mutex.synchronize do
+          name = name.to_s
+          return @data[name] if @data.include?(name)
+          match = name.match(/\A(.*?)([?=]?)\Z/)
+          case match[2]
+          when "="
+            @data[match[1]] = args.first
+          when "?"
+            !!@data[match[1]]
+          end
         end
       end
     end