diff --git a/NEWS.md b/NEWS.md
index 46a0b5c..b51023f 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -11,6 +11,7 @@ Development release.
 ## Bug fixes
 
 - Added a missing `@family` tag to the `pcens` functions. This omission resulted in the Weibull analytical solution not being visible in the package documentation.
+- Changed a call to `size()` to use `num_elements()` instead as an underlying type conversion was causing issues on some platforms.
 - Changed `D` to be of type real in order to support infinite `relative_obs_time`.
 
 # primarycensored 1.0.0
diff --git a/inst/stan/functions/primarycensored.stan b/inst/stan/functions/primarycensored.stan
index c424dde..5534b7a 100644
--- a/inst/stan/functions/primarycensored.stan
+++ b/inst/stan/functions/primarycensored.stan
@@ -37,8 +37,8 @@ real primarycensored_cdf(data real d, int dist_id, array[] real params,
   } else {
     // Use numerical integration for other cases
     real lower_bound = max({d - pwindow, 1e-6});
-    array[size(params) + size(primary_params)] real theta = append_array(params, primary_params);
-    array[4] int ids = {dist_id, primary_id, size(params), size(primary_params)};
+    array[num_elements(params) + num_elements(primary_params)] real theta = append_array(params, primary_params);
+    array[4] int ids = {dist_id, primary_id, num_elements(params), num_elements(primary_params)};
 
     vector[1] y0 = rep_vector(0.0, 1);
     result = ode_rk45(primarycensored_ode, y0, lower_bound, {d}, theta, {d, pwindow}, ids)[1, 1];