diff --git a/src/sample.c b/src/sample.c
index 91fb0a7..abce9b9 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -3432,7 +3432,7 @@
 	value = fix_tag_value(ist2(smp->data.u.str.area, smp->data.u.str.data),
 			      arg_p[0].data.sint);
 	if (!istlen(value)) {
-		if (!isttest(value)) {
+		if (isttest(value)) {
 			/* value != IST_NULL, need more data */
 			smp->flags |= SMP_F_MAY_CHANGE;
 		}