diff --git a/lightningd/subd.c b/lightningd/subd.c
index 49ad2f47c..770a7acc5 100644
--- a/lightningd/subd.c
+++ b/lightningd/subd.c
@@ -30,6 +30,7 @@
 
 static bool move_fd(int from, int to)
 {
+	assert(from >= 0);
 	if (dup2(from, to) == -1)
 		return false;
 	close(from);