diff --git a/tunnel/tcp.go b/tunnel/tcp.go
index 569db02..beb8167 100644
--- a/tunnel/tcp.go
+++ b/tunnel/tcp.go
@@ -79,12 +79,19 @@ func copyBuffer(dst io.Writer, src io.Reader) error {
 	defer pool.Put(buf)
 
 	_, err := io.CopyBuffer(dst, src, buf)
-	if ne, ok := err.(net.Error); ok && ne.Timeout() {
-		return nil /* ignore I/O timeout */
-	} else if errors.Is(err, syscall.EPIPE) {
-		return nil /* ignore broken pipe */
-	} else if errors.Is(err, syscall.ECONNRESET) {
-		return nil /* ignore connection reset by peer */
+	if err != nil && !isIgnorable(err) {
+		return err
 	}
-	return err
+	return nil
+}
+
+func isIgnorable(err error) bool {
+	if ne, ok := err.(net.Error); ok && ne.Timeout() {
+		return true /* ignore I/O timeout */
+	} else if errors.Is(err, syscall.EPIPE) {
+		return true /* ignore broken pipe */
+	} else if errors.Is(err, syscall.ECONNRESET) {
+		return true /* ignore connection reset by peer */
+	}
+	return false
 }