diff --git a/mqttpacket.cpp b/mqttpacket.cpp
index 52a8481..f935412 100644
--- a/mqttpacket.cpp
+++ b/mqttpacket.cpp
@@ -44,6 +44,7 @@ MqttPacket::MqttPacket(CirBuf &buf, size_t packet_len, size_t fixed_header_lengt
     unsigned char _packetType = (first_byte & 0xF0) >> 4;
     packetType = (PacketType)_packetType;
     pos += fixed_header_length;
+    externallyReceived = true;
 }
 
 MqttPacket::MqttPacket(const ConnAck &connAck) :
@@ -123,7 +124,7 @@ size_t MqttPacket::getRequiredSizeForPublish(const ProtocolVersion protocolVersi
 MqttPacket::MqttPacket(const ProtocolVersion protocolVersion, const Publish &_publish) :
     bites(getRequiredSizeForPublish(protocolVersion, _publish))
 {
-    if (publishData.topic.length() > 0xFFFF)
+    if (_publish.topic.length() > 0xFFFF)
     {
         throw ProtocolError("Topic path too long.");
     }
@@ -1301,6 +1302,7 @@ const Publish &MqttPacket::getPublishData()
 bool MqttPacket::containsClientSpecificProperties() const
 {
     assert(packetType == PacketType::PUBLISH);
+    assert(this->externallyReceived);
 
     if (protocolVersion <= ProtocolVersion::Mqtt311 || !publishData.propertyBuilder)
         return false;
diff --git a/mqttpacket.h b/mqttpacket.h
index 856a718..1b2795d 100644
--- a/mqttpacket.h
+++ b/mqttpacket.h
@@ -36,6 +36,13 @@ License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
 #include "variablebyteint.h"
 #include "mqtt5properties.h"
 
+/**
+ * @brief The MqttPacket class represents incoming and outgonig packets.
+ *
+ * Be sure to understand the 'externallyReceived' member. See in-code documentation.
+ *
+ * TODO: I could perhaps make a subclass for the externally received one.
+ */
 class MqttPacket
 {
 #ifdef TESTING
@@ -55,6 +62,12 @@ class MqttPacket
     size_t payloadStart = 0;
     size_t payloadLen = 0;
     bool hasTopicAlias = false;
+
+    // It's important to understand that this class is used for incoming packets as well as new outgoing packets. When we create
+    // new outgoing packets, we generally know exactly who it's for and the information is only stored in this->bites. So, the
+    // publishData and fields like hasTopicAlias are invalid in those cases.
+    bool externallyReceived = false;
+
     Logger *logger = Logger::getInstance();
 
     char *readBytes(size_t length);