diff --git a/src/strings.c b/src/strings.c
index 7a99cd9..0313e74 100644
--- a/src/strings.c
+++ b/src/strings.c
@@ -898,6 +898,7 @@
 	    break;
 	len = (int)STRLEN(tv.vval.v_string);
 
+	newtv.v_type = VAR_UNKNOWN;
 	set_vim_var_nr(VV_KEY, idx);
 	if (filter_map_one(&tv, expr, filtermap, &newtv, &rem) == FAIL
 		|| did_emsg)