diff --git a/src/runtime/heapdump.go b/src/runtime/heapdump.go
index c317b5f969..f7d7aac2a7 100644
--- a/src/runtime/heapdump.go
+++ b/src/runtime/heapdump.go
@@ -474,7 +474,7 @@ func dumpobjs() {
 			throw("freemark array doesn't have enough entries")
 		}
 
-		for freeIndex := s.freeindex; freeIndex < s.nelems; freeIndex++ {
+		for freeIndex := uintptr(0); freeIndex < s.nelems; freeIndex++ {
 			if s.isFree(freeIndex) {
 				freemark[freeIndex] = true
 			}
diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go
index 3363cd0682..7171d3adbd 100644
--- a/src/runtime/mbitmap.go
+++ b/src/runtime/mbitmap.go
@@ -264,7 +264,11 @@ func (s *mspan) nextFreeIndex() uintptr {
 	return result
 }
 
+// isFree returns whether the index'th object in s is unallocated.
 func (s *mspan) isFree(index uintptr) bool {
+	if index < s.freeindex {
+		return false
+	}
 	whichByte := index / 8
 	whichBit := index % 8
 	byteVal := *addb(s.allocBits, whichByte)