diff --git a/examples/nanocoap_server/coap_handler.c b/examples/nanocoap_server/coap_handler.c
index 8b179a3a07..09ea785ed2 100644
--- a/examples/nanocoap_server/coap_handler.c
+++ b/examples/nanocoap_server/coap_handler.c
@@ -150,6 +150,10 @@ ssize_t _sha256_handler(coap_pkt_t* pkt, uint8_t *buf, size_t len, coap_request_
     }
 
     ssize_t reply_len = coap_build_reply(pkt, result, buf, len, 0);
+    if (reply_len <= 0) {
+        return reply_len;
+    }
+
     uint8_t *pkt_pos = (uint8_t*)pkt->hdr + reply_len;
     if (blockwise) {
         pkt_pos += coap_opt_put_block1_control(pkt_pos, 0, &block1);
diff --git a/tests/riotboot_flashwrite/coap_handler.c b/tests/riotboot_flashwrite/coap_handler.c
index 6ac1352157..50fd24c275 100644
--- a/tests/riotboot_flashwrite/coap_handler.c
+++ b/tests/riotboot_flashwrite/coap_handler.c
@@ -66,6 +66,10 @@ ssize_t _flashwrite_handler(coap_pkt_t* pkt, uint8_t *buf, size_t len, coap_requ
     }
 
     ssize_t reply_len = coap_build_reply(pkt, result, buf, len, 0);
+    if (reply_len <= 0) {
+        return reply_len;
+    }
+
     uint8_t *pkt_pos = (uint8_t*)pkt->hdr + reply_len;
     pkt_pos += coap_put_block1_ok(pkt_pos, &block1, 0);