diff --git a/HD44780.cpp b/HD44780.cpp
index f293e62..07bbf87 100644
--- a/HD44780.cpp
+++ b/HD44780.cpp
@@ -44,6 +44,7 @@ m_dmr(false)
 {
 	assert(rows > 1U);
 	assert(cols > 15U);
+	assert(callsign != NULL);
 }
 
 CHD44780::~CHD44780()
@@ -212,6 +213,7 @@ void CHD44780::clearDStar()
 void CHD44780::writeDMR(unsigned int slotNo, const char* src, bool group, const char* dst, const char* type)
 {
 	assert(src != NULL);
+	assert(dst != NULL);
 	assert(type != NULL);
 
 	if (!m_dmr) {
diff --git a/Nextion.cpp b/Nextion.cpp
index 3670f05..f3a8015 100644
--- a/Nextion.cpp
+++ b/Nextion.cpp
@@ -30,6 +30,7 @@ m_serial(port, SERIAL_9600),
 m_brightness(brightness),
 m_mode(MODE_IDLE)
 {
+	assert(callsign != NULL);
 	assert(brightness >= 0U && brightness <= 100U);
 }
 
@@ -127,6 +128,7 @@ void CNextion::clearDStar()
 void CNextion::writeDMR(unsigned int slotNo, const char* src, bool group, const char* dst, const char* type)
 {
 	assert(src != NULL);
+	assert(dst != NULL);
 	assert(type != NULL);
 
 	if (m_mode != MODE_DMR) {
diff --git a/TFTSerial.cpp b/TFTSerial.cpp
index 775f2e6..b6e608e 100644
--- a/TFTSerial.cpp
+++ b/TFTSerial.cpp
@@ -51,6 +51,7 @@ m_serial(port, SERIAL_9600),
 m_brightness(brightness),
 m_mode(MODE_IDLE)
 {
+	assert(callsign != NULL);
 	assert(brightness >= 0U && brightness <= 100U);
 }
 
@@ -190,6 +191,7 @@ void CTFTSerial::clearDStar()
 void CTFTSerial::writeDMR(unsigned int slotNo, const char* src, bool group, const char* dst, const char* type)
 {
 	assert(src != NULL);
+	assert(dst != NULL);
 	assert(type != NULL);
 
 	if (m_mode != MODE_DMR) {