diff --git a/src/gentypes.jl b/src/gentypes.jl
index 02bd0df..1c32a06 100644
--- a/src/gentypes.jl
+++ b/src/gentypes.jl
@@ -12,8 +12,13 @@ struct Top end
 get_type(NT, k) = hasfield(NT, k) ? fieldtype(NT, k) : Nothing
 
 # unify two types to a single type
+function promoteunion(T, S)
+    new = promote_type(T, S)
+    return isabstracttype(new) ? Union{T, S} : new
+end
+
 unify(a, b) = unify(b, a)
-unify(a::Type{T}, b::Type{S}) where {T,S} = Base.promote_typejoin(T, S)
+unify(a::Type{T}, b::Type{S}) where {T,S} = promoteunion(T, S)
 unify(a::Type{T}, b::Type{S}) where {T,S<:T} = T
 unify(a::Type{Top}, b::Type{T}) where {T} = T
 
diff --git a/test/gentypes.jl b/test/gentypes.jl
index e0e4a13..9cf6897 100644
--- a/test/gentypes.jl
+++ b/test/gentypes.jl
@@ -252,6 +252,7 @@
 
         @test !(JSONTypes.Root.mutable)
         @test parsed[1].c.d == 4
+        @test fieldtype(JSONTypes.Root, 1) == Union{Int64, String}
     end
 
     @testset "Raw Types" begin