diff --git a/src/acme.c b/src/acme.c
index 6b4dcd2af..7875f11e8 100644
--- a/src/acme.c
+++ b/src/acme.c
@@ -1532,7 +1532,7 @@ int acme_res_certificate(struct task *task, struct acme_ctx *ctx, char **errmsg)
 
 	hdrs = hc->res.hdrs;
 
-	for (hdr = hdrs; isttest(hdr->v); hdr++) {
+	for (hdr = hdrs; hdrs && isttest(hdr->v); hdr++) {
 		if (isteqi(hdr->n, ist("Replay-Nonce"))) {
 			istfree(&ctx->nonce);
 			ctx->nonce = istdup(hdr->v);
@@ -1745,7 +1745,7 @@ int acme_res_finalize(struct task *task, struct acme_ctx *ctx, char **errmsg)
 
 	hdrs = hc->res.hdrs;
 
-	for (hdr = hdrs; isttest(hdr->v); hdr++) {
+	for (hdr = hdrs; hdrs && isttest(hdr->v); hdr++) {
 		if (isteqi(hdr->n, ist("Replay-Nonce"))) {
 			istfree(&ctx->nonce);
 			ctx->nonce = istdup(hdr->v);
@@ -1846,7 +1846,7 @@ enum acme_ret acme_res_challenge(struct task *task, struct acme_ctx *ctx, struct
 
 	TRACE_DATA(__FUNCTION__, ACME_EV_RES, ctx, NULL, &hc->res.buf);
 
-	for (hdr = hdrs; isttest(hdr->v); hdr++) {
+	for (hdr = hdrs; hdrs && isttest(hdr->v); hdr++) {
 		if (isteqi(hdr->n, ist("Replay-Nonce"))) {
 			istfree(&ctx->nonce);
 			ctx->nonce = istdup(hdr->v);
@@ -1973,7 +1973,7 @@ int acme_res_auth(struct task *task, struct acme_ctx *ctx, struct acme_auth *aut
 
 	hdrs = hc->res.hdrs;
 
-	for (hdr = hdrs; isttest(hdr->v); hdr++) {
+	for (hdr = hdrs; hdrs && isttest(hdr->v); hdr++) {
 		if (isteqi(hdr->n, ist("Replay-Nonce"))) {
 			istfree(&ctx->nonce);
 			ctx->nonce = istdup(hdr->v);
@@ -2289,7 +2289,7 @@ int acme_res_neworder(struct task *task, struct acme_ctx *ctx, char **errmsg)
 
 	hdrs = hc->res.hdrs;
 
-	for (hdr = hdrs; isttest(hdr->v); hdr++) {
+	for (hdr = hdrs; hdrs && isttest(hdr->v); hdr++) {
 		if (isteqi(hdr->n, ist("Replay-Nonce"))) {
 			istfree(&ctx->nonce);
 			ctx->nonce = istdup(hdr->v);
@@ -2468,7 +2468,7 @@ int acme_res_account(struct task *task, struct acme_ctx *ctx, int newaccount, ch
 
 	hdrs = hc->res.hdrs;
 
-	for (hdr = hdrs; isttest(hdr->v); hdr++) {
+	for (hdr = hdrs; hdrs && isttest(hdr->v); hdr++) {
 		if (isteqi(hdr->n, ist("Location"))) {
 			istfree(&ctx->kid);
 			ctx->kid = istdup(hdr->v);
@@ -2535,7 +2535,7 @@ int acme_nonce(struct task *task, struct acme_ctx *ctx, char **errmsg)
 
 	hdrs = hc->res.hdrs;
 
-	for (hdr = hdrs; isttest(hdr->v); hdr++) {
+	for (hdr = hdrs; hdrs && isttest(hdr->v); hdr++) {
 		if (isteqi(hdr->n, ist("Replay-Nonce"))) {
 			istfree(&ctx->nonce);
 			ctx->nonce = istdup(hdr->v);