diff --git a/src/mzscheme/src/mzmark.c b/src/mzscheme/src/mzmark.c
index ddc17fc0a4..cb2e36a299 100644
--- a/src/mzscheme/src/mzmark.c
+++ b/src/mzscheme/src/mzmark.c
@@ -3637,7 +3637,7 @@ static int mark_input_fd_FIXUP(void *p) {
 
 #endif
 
-#if defined(UNIX_PROCESSES)
+#if defined(UNIX_PROCESSES) && !(defined(MZ_USE_PLACES) && defined(MZ_PRECISE_GC))
 static int mark_system_child_SIZE(void *p) {
   return
   gcBYTES_TO_WORDS(sizeof(System_Child));