diff --git a/src/tracker/back.jl b/src/tracker/back.jl
index 06f62e5d..9ca0377c 100644
--- a/src/tracker/back.jl
+++ b/src/tracker/back.jl
@@ -79,14 +79,14 @@ function Base.show(io::IO, ps::Params)
 end
 
 struct Grads
-  grads::ObjectIdDict
+  grads::IdDict{Any,Any}
 end
 
 Base.show(io::IO, ps::Grads) = println(io, "Grads(...)")
 
-Grads() = Grads(ObjectIdDict())
+Grads() = Grads(IdDict())
 
-Grads(ps::Params) = Grads(ObjectIdDict(tracker(p) => init_grad(data(p)) for p in ps))
+Grads(ps::Params) = Grads(IdDict(tracker(p) => init_grad(data(p)) for p in ps))
 
 Base.getindex(g::Grads, x::Tracked) = g.grads[x]
 function Base.getindex(g::Grads, x)
diff --git a/src/tracker/idset.jl b/src/tracker/idset.jl
index 68d1eea1..0d5fade9 100644
--- a/src/tracker/idset.jl
+++ b/src/tracker/idset.jl
@@ -1,6 +1,6 @@
 struct IdSet{T} <: AbstractSet{T}
-  dict::ObjectIdDict
-  IdSet{T}() where T = new(ObjectIdDict())
+  dict::IdDict{T,Nothing}
+  IdSet{T}() where T = new(IdDict{T,Nothing}())
 end
 
 Base.eltype{T}(::IdSet{T}) = T
diff --git a/src/treelike.jl b/src/treelike.jl
index e65ac41a..e4c4e33f 100644
--- a/src/treelike.jl
+++ b/src/treelike.jl
@@ -16,7 +16,7 @@ end
 
 isleaf(x) = isempty(children(x))
 
-function mapleaves(f, x; cache = ObjectIdDict())
+function mapleaves(f, x; cache = IdDict())
   haskey(cache, x) && return cache[x]
   cache[x] = isleaf(x) ? f(x) : mapchildren(x -> mapleaves(f, x, cache = cache), x)
 end