diff --git a/pkgs/racket-test-core/tests/racket/foreign-test.c b/pkgs/racket-test-core/tests/racket/foreign-test.c
index 60e59c085b..389a98d8b4 100644
--- a/pkgs/racket-test-core/tests/racket/foreign-test.c
+++ b/pkgs/racket-test-core/tests/racket/foreign-test.c
@@ -325,7 +325,7 @@ X int check_multiple_of_ten(int v) {
   }
 }
 
-int sum_after_callback(int *a, int n, void (*cb)()) {
+X int sum_after_callback(int *a, int n, void (*cb)()) {
   int i, s = 0;
 
   cb();