--- libmalloc/libmalloc-521.120.7/tests/scribble_tests.c
+++ libmalloc/libmalloc-646.0.13/tests/scribble_tests.c
@@ -29,6 +29,7 @@
 T_DECL(malloc_scribble_check, "check MallocScribble works",
 	T_META_TAG_XZONE,
 	T_META_ENVVAR("MallocScribble=1"),
+	T_META_ENVVAR("MallocProbGuard=0"),  // rdar://121458833
 	T_META_CHECK_LEAKS(false))
 {
 	// Ensure that TINY allocations are scribbled on allocation