diff --git a/src/core/ad.jl b/src/core/ad.jl
index a042ea3aa..7f767171e 100644
--- a/src/core/ad.jl
+++ b/src/core/ad.jl
@@ -55,12 +55,12 @@ gradient(vi::VarInfo, model::Function, spl::Union{Void, Sampler}) = begin
       vals = getval(vi, vns[i])
       if vns[i] in vn_chunk        # for each variable to compute gradient in this round
         for i = 1:l
-          vi[range[i]] = ForwardDiff.Dual{CHUNKSIZE, Float64}(realpart(vals[i]), SEEDS[dim_count])
+          vi[range[i]] = ForwardDiff.Dual{Void, Float64, CHUNKSIZE}(realpart(vals[i]), SEEDS[dim_count])
           dim_count += 1      # count
         end
       else                    # for other varilables (no gradient in this round)
         for i = 1:l
-          vi[range[i]] = ForwardDiff.Dual{CHUNKSIZE, Float64}(realpart(vals[i]))
+          vi[range[i]] = ForwardDiff.Dual{Void, Float64, CHUNKSIZE}(realpart(vals[i]))
         end
       end
     end