diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 0e7cd11..1bb709c 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -719,6 +719,10 @@
 // gcDumpObject dumps the contents of obj for debugging and marks the
 // field at byte offset off in obj.
 func gcDumpObject(label string, obj, off uintptr) {
+	if obj < mheap_.arena_start || obj >= mheap_.arena_used {
+		print(label, "=", hex(obj), " is not a heap object\n")
+		return
+	}
 	k := obj >> _PageShift
 	x := k
 	x -= mheap_.arena_start >> _PageShift