diff --git a/benchmarks/dvmp/analysis/vm_invar.cxx b/benchmarks/dvmp/analysis/vm_invar.cxx
index 2b043f847670bce9b48f4afd29af338cc9a970fd..adf362aa44e21e5261f2d2b42af49aa6cc2560b2 100644
--- a/benchmarks/dvmp/analysis/vm_invar.cxx
+++ b/benchmarks/dvmp/analysis/vm_invar.cxx
@@ -150,7 +150,7 @@ int vm_invar(const std::string& config_name)
   
   TH1D* h_sim[4];
   for(int i = 0 ; i < 4 ; i++){
-    auto h_tmp = d_im.Histo1D({(VarName[i]+"test").c_str(), histTitle[i].c_str(), 50, hist_range_l[i], hist_range_h[i]}, RawHist[i].c_str());       //directly quote the string
+    auto h_tmp = d_im.Histo1D({(fmt::format("{}_test", VarName[i])).c_str(), histTitle[i].c_str(), 50, hist_range_l[i], hist_range_h[i]}, RawHist[i].c_str());       //directly quote the string
     h_sim[i] = &(*h_tmp);
     h_sim[i] = (TH1D*)h_sim[i]->Clone();
   }