diff --git a/mongoose.c b/mongoose.c
index 110b18b0..fc4f920f 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -4333,7 +4333,7 @@ void mg_mqtt_login(struct mg_connection *c, const struct mg_mqtt_opts *opts) {
     total_len += 2 + (uint32_t) opts->pass.len;
     hdr[7] |= MQTT_HAS_PASSWORD;
   }
-  if (opts->topic.len > 0 && opts->message.len > 0) {
+  if (opts->topic.len > 0) { // allow zero-length msgs, message.len is size_t
     total_len += 4 + (uint32_t) opts->topic.len + (uint32_t) opts->message.len;
     hdr[7] |= MQTT_HAS_WILL;
   }
diff --git a/src/mqtt.c b/src/mqtt.c
index 8531424b..c46b4f9a 100644
--- a/src/mqtt.c
+++ b/src/mqtt.c
@@ -277,7 +277,7 @@ void mg_mqtt_login(struct mg_connection *c, const struct mg_mqtt_opts *opts) {
     total_len += 2 + (uint32_t) opts->pass.len;
     hdr[7] |= MQTT_HAS_PASSWORD;
   }
-  if (opts->topic.len > 0 && opts->message.len > 0) {
+  if (opts->topic.len > 0) { // allow zero-length msgs, message.len is size_t
     total_len += 4 + (uint32_t) opts->topic.len + (uint32_t) opts->message.len;
     hdr[7] |= MQTT_HAS_WILL;
   }