diff --git a/README.md b/README.md
index 299903fc..d7d376b2 100644
--- a/README.md
+++ b/README.md
@@ -649,6 +649,7 @@ If you're using the `X-SourceMap` header instead, you can just omit `sourceMap.u
 - `arrows` (default `true`) -- Converts `()=>{return x}` to `()=>x`. Class
   and object literal methods will also be converted to arrow expressions if
   the resultant code is shorter: `m(){return x}` becomes `m:()=>x`.
+  This transform requires that the `ecma` compress option is set to `6` or greater.
 
 - `unsafe_arrows` (default `false`) -- Convert ES5 style anonymous function
   expressions to arrow functions if the function body does not reference `this`.