diff --git a/prototype/Doubt/Cofree.swift b/prototype/Doubt/Cofree.swift
index 670f2eefe..7aa71d94b 100644
--- a/prototype/Doubt/Cofree.swift
+++ b/prototype/Doubt/Cofree.swift
@@ -37,7 +37,7 @@ extension Cofree {
 		}
 	}
 
-	public func extend<Other>(transform: Cofree -> Other) -> Cofree<A, Other> {
+	public func extend<Other>(@noescape transform: Cofree -> Other) -> Cofree<A, Other> {
 		return .Unroll(transform(self), unwrap.map { $0.extend(transform) })
 	}