diff --git a/src/proto_http.c b/src/proto_http.c
index d566bcc81..01fe62d09 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -9748,6 +9748,9 @@ smp_prefetch_http(struct proxy *px, struct session *s, void *l7, unsigned int op
 	return 1;
 }
 
+/* Note: these functinos *do* modify the sample. Even in case of success, at
+ * least the type and uint value are modified.
+ */
 #define CHECK_HTTP_MESSAGE_FIRST() \
 	do { int r = smp_prefetch_http(px, l4, l7, opt, args, smp, 1); if (r <= 0) return r; } while (0)
 
diff --git a/src/sample.c b/src/sample.c
index 9f22ef971..3a0f3fbbc 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -905,7 +905,7 @@ struct sample *sample_process(struct proxy *px, struct session *l4, void *l7,
 
 	if (p == NULL) {
 		p = &temp_smp;
-		p->flags = 0;
+		memset(p, 0, sizeof(*p));
 	}
 
 	if (!expr->fetch->process(px, l4, l7, opt, expr->arg_p, p, expr->fetch->kw))
@@ -1160,7 +1160,8 @@ struct sample *sample_fetch_string(struct proxy *px, struct session *l4, void *l
 {
 	struct sample *smp = &temp_smp;
 
-	smp->flags = 0;
+	memset(smp, 0, sizeof(*smp));
+
 	if (!sample_process(px, l4, l7, opt, expr, smp)) {
 		if ((smp->flags & SMP_F_MAY_CHANGE) && !(opt & SMP_OPT_FINAL))
 			return smp;