Error for multinomial distribution


I am using multinomial distribution for my data and model,

In my model, I use nextstate[i,] ~ multinomial(prob).

I always have a note saying that: “No matches for: Available argument signatures for multinomial: real return type required for probability function”.

however, I have no idea how to fix it, can someone help with this? Thank you.

My model is pasted as follows:

data{//input data

  int<lower = 1> N; //nb data row 
  int<lower = 1> S; //nb of states
  real times[N]; //time point
  int currentstate[N]; //current state
  real nextstate[N, S];//next state matrix

matrix[S, S] Vmatrix; //from is row; to is collum


transformed parameters {
  matrix[S,S] Vmatrix_trans;
  Vmatrix_trans = Vmatrix;
  Vmatrix_trans[5, ] = [0,0,0,0,0];

  matrix[S, S] ProbTrans; 
  real prob[S];

  for(i in 1:N){
  ProbTrans = matrix_exp(times[i]*Vmatrix_trans);
  prob = to_array_1d(ProbTrans[currentstate[i], ]);
  nextstate[i,] ~ multinomial(prob);
  //priors ...

The multinomial distribution is only defined for integer outcomes, but you’ve declared nextstate as a real.

Try changing the data definition to int nextstate[N, S]