diff --git a/src/immer.js.flow b/src/immer.js.flow
index 221aecde..2c325af6 100644
--- a/src/immer.js.flow
+++ b/src/immer.js.flow
@@ -71,6 +71,8 @@ declare export default IProduce
 
 declare export var nothing: typeof undefined
 
+declare export var immerable: Symbol
+
 /**
  * Automatically freezes any state trees generated by immer.
  * This protects against accidental modifications of the state tree outside of an immer function.