diff --git a/src/samplers/pgibbs.jl b/src/samplers/pgibbs.jl
index 21dad8d31b..11669cfd98 100644
--- a/src/samplers/pgibbs.jl
+++ b/src/samplers/pgibbs.jl
@@ -50,6 +50,7 @@ step(model::Function, spl::Sampler{PG}, vi::VarInfo, _::Bool) = step(model, spl,
 step(model::Function, spl::Sampler{PG}, vi::VarInfo) = begin
   particles = ParticleContainer{TraceR}(model)
 
+  vi.index = 0; vi.num_produce = 0;  # We need this line cause fork2 deepcopy `vi`. 
   ref_particle = isempty(vi) ?
                  nothing :
                  fork2(TraceR(model, spl, vi))