diff --git a/src/popupwin.c b/src/popupwin.c
index de60485..64bb0b5 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -2382,6 +2382,8 @@
     typval_T	rettv;
     typval_T	argv[3];
 
+    rettv.v_type = VAR_UNKNOWN;
+
     argv[0].v_type = VAR_NUMBER;
     argv[0].vval.v_number = (varnumber_T)wp->w_id;