diff --git a/process.c b/process.c
index 848d94df20..2c9ef5021a 100644
--- a/process.c
+++ b/process.c
@@ -2546,7 +2546,9 @@ rb_pid_t
 rb_fork(int *status, int (*chfunc)(void*), void *charg, VALUE fds)
 {
     if (chfunc) {
-        struct chfunc_wrapper_t warg = { chfunc, charg };
+        struct chfunc_wrapper_t warg;
+        warg.chfunc = chfunc;
+        warg.arg = charg;
         return rb_fork_err(status, chfunc_wrapper, &warg, fds, NULL, 0);
     }
     else {