diff --git a/src/window.c b/src/window.c
index eed0ea9..2ef3198 100644
--- a/src/window.c
+++ b/src/window.c
@@ -3806,6 +3806,7 @@
 	for (ptp = first_tabpage; ptp != NULL && ptp->tp_next != tab;
 							    ptp = ptp->tp_next)
 	    ;
+	assert(ptp != NULL);
 	ptp->tp_next = tab->tp_next;
     }