diff --git a/prototype/Doubt/Cofree.swift b/prototype/Doubt/Cofree.swift
index 7aa71d94b..f2a286112 100644
--- a/prototype/Doubt/Cofree.swift
+++ b/prototype/Doubt/Cofree.swift
@@ -20,7 +20,7 @@ public enum Cofree<A, B> {
 // MARK: - Functor
 
 extension Cofree {
-	public func map<Other>(transform: B -> Other) -> Cofree<A, Other> {
+	public func map<Other>(@noescape transform: B -> Other) -> Cofree<A, Other> {
 		return .Unroll(transform(extract), unwrap.map { $0.map(transform) })
 	}
 }