diff --git a/src/mzscheme/gc2/compact.c b/src/mzscheme/gc2/compact.c
index c9c7eb28ef..dd4e29f37b 100644
--- a/src/mzscheme/gc2/compact.c
+++ b/src/mzscheme/gc2/compact.c
@@ -2622,6 +2622,8 @@ void protect_old_mpages()
 
 #if GENERATIONS
 
+static int designate_modified(void *p);
+
 static int designate_modified_maybe(void *p, int no_barrier_ok)
 {
   unsigned long g = ((unsigned long)p >> MAPS_SHIFT);