diff --git a/mem/file.go b/mem/file.go
index 7af2fb5..f99590e 100644
--- a/mem/file.go
+++ b/mem/file.go
@@ -193,8 +193,11 @@ func (f *File) Read(b []byte) (n int, err error) {
 }
 
 func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
+	prev := atomic.LoadInt64(&f.at)
 	atomic.StoreInt64(&f.at, off)
-	return f.Read(b)
+	n, err = f.Read(b)
+	atomic.StoreInt64(&f.at, prev)
+	return
 }
 
 func (f *File) Truncate(size int64) error {