diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 5a81d9f..f6e9d47 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -912,7 +912,7 @@ static inline abi_long copy_to_user_fdset(abi_ulong target_fds_addr,
     for (i = 0; i < nw; i++) {
         v = 0;
         for (j = 0; j < TARGET_ABI_BITS; j++) {
-            v |= ((FD_ISSET(k, fds) != 0) << j);
+            v |= (abi_ulong) (FD_ISSET(k, fds) != 0) << j;
             k++;
         }
         __put_user(v, &target_fds[i]);