diff --git a/base/generator.jl b/base/generator.jl
index b0f7e32d0b22f..bd531482229b4 100644
--- a/base/generator.jl
+++ b/base/generator.jl
@@ -51,6 +51,7 @@ length(g::Generator) = length(g.iter)
 size(g::Generator) = size(g.iter)
 axes(g::Generator) = axes(g.iter)
 ndims(g::Generator) = ndims(g.iter)
+keys(g::Generator) = OneTo(length(g))
 
 
 ## iterator traits
diff --git a/test/functional.jl b/test/functional.jl
index f2d6d4a898ede..d2f5d30572639 100644
--- a/test/functional.jl
+++ b/test/functional.jl
@@ -219,3 +219,6 @@ end
 let (:)(a,b) = (i for i in Base.:(:)(1,10) if i%2==0)
     @test Int8[ i for i = 1:2 ] == [2,4,6,8,10]
 end
+
+@test keys(sin(x) for x in 0.0:0.5:9.9) == 1:20
+