concatenate two recursive linked list in C

I want to concatenate two linked lists L1 and L2. but it didn't work when i called the concat2 on my main and only showed random numbers and ended the process. can anyone help me? i'm new to c :( thanks in advance

Here's my code:

List Concat2 (List L1, List L2){
   if(IsEmpty(L2)){
      return Copy(L1);
   }else{
      return inslast(Concat2(L1,Head(L2)),LastElmt(L2));
   }
} 

And here's my code for Head and LastElmt:

List Head (List L){
   if(IsEmpty(L)){
      return Null;
   }else{
      return info(L);
   }
}

int LastElmt(List L){
   address Last;

   if(IsEmpty(L)){
      return info(L);
   }else{
      Last = L;
      while(next(Last)!=Null){
          Last = next(Last);
      }
      return info(Last);
   }
}

for example List L1: 1 2 3 List L2: 4 5 6 expected output: 1 2 3 4 5 6 my code output: process returned -1073741819 (0xC0000005)