diff --git a/iptables/iptables.go b/iptables/iptables.go
index 2d6737a..399d10b 100644
--- a/iptables/iptables.go
+++ b/iptables/iptables.go
@@ -425,6 +425,7 @@ func (ipt *IPTables) runWithOutput(args []string, stdout io.Writer) error {
 		}
 		ul, err := fmu.tryLock()
 		if err != nil {
+			syscall.Close(fmu.fd)
 			return err
 		}
 		defer ul.Unlock()