diff --git a/internal/unionstore/pipelined_memdb.go b/internal/unionstore/pipelined_memdb.go
index cd72c8698..69b39ba22 100644
--- a/internal/unionstore/pipelined_memdb.go
+++ b/internal/unionstore/pipelined_memdb.go
@@ -411,7 +411,14 @@ func (p *PipelinedMemDB) onMemChange() {
 
 // Mem returns the memory usage of MemBuffer.
 func (p *PipelinedMemDB) Mem() uint64 {
-	return p.memDB.Mem() + p.flushingMemDB.Mem()
+	var mem uint64
+	if p.memDB != nil {
+		mem += p.memDB.Mem()
+	}
+	if p.flushingMemDB != nil {
+		mem += p.flushingMemDB.Mem()
+	}
+	return mem
 }
 
 type errIterator struct {