diff --git a/head.go b/head.go
index 704c0b222b..dd441f7973 100644
--- a/head.go
+++ b/head.go
@@ -260,9 +260,12 @@ Outer:
 		return err
 	}
 
+	// Map is accessed in other places also, so protect it.
+	h.mtx.Lock()
 	for k, v := range newStones {
 		h.tombstones[k] = h.tombstones[k].add(v[0])
 	}
+	h.mtx.Unlock()
 
 	h.meta.NumTombstones = int64(len(h.tombstones))
 	return nil