diff --git a/src/index.js.flow b/src/index.js.flow
index 4216deb..f4de68b 100644
--- a/src/index.js.flow
+++ b/src/index.js.flow
@@ -4,9 +4,10 @@ export type Version = 0 | 1
 export type Codec = string
 export type Multihash = Buffer
 export type BaseEncodedString = string
+export type MultibaseName = string
 
 declare class CID<a> {
-  constructor(Version, Codec, Multihash): void;
+  constructor(Version, Codec, Multihash, multibaseName?:MultibaseName): void;
   constructor(BaseEncodedString): void;
   constructor(Buffer): void;