diff --git a/inst/stan/include/distributions.stan b/inst/stan/include/distributions.stan
index 4368274..f8d8d31 100644
--- a/inst/stan/include/distributions.stan
+++ b/inst/stan/include/distributions.stan
@@ -130,19 +130,19 @@ real surv_weibull_lpdf (real t, real d, real shape, real scale) {
   return log_lik;
 }
 
-// take additional arguments for integration
-real weibull_survival_1d(real t, real xc, array[] real theta, array[] real x_r, array[] int x_i) {
+// Define the function to be integrated
+real weibull_survival_1d(real t, real xc, vector theta, data vector x_r, data int x_i) {
   real shape = theta[1];
   real scale = theta[2];
   return exp(-pow(t / scale, shape));
 }
 
-// restricted mean survival time
+// Restricted mean survival time
 real rmst_weibull(real shape, real scale, real tmax) {
   real rmst;
-  array[2] real theta = {shape, scale};
-  array[0] real x_r;
-  array[0] int x_i;
+  vector[2] theta = [shape, scale];
+  vector[0] x_r;  // zero-sized vector
+  int[0] x_i;     // zero-sized int array
 
   // integrate survival function from 0 to tmax
   rmst = integrate_1d(weibull_survival_1d, 0, tmax, theta, x_r, x_i, 1e-8);