diff --git a/src/protocols/AFSK/AFSK.cpp b/src/protocols/AFSK/AFSK.cpp
index a8453925..e2a90641 100644
--- a/src/protocols/AFSK/AFSK.cpp
+++ b/src/protocols/AFSK/AFSK.cpp
@@ -6,6 +6,10 @@ AFSKClient::AFSKClient(PhysicalLayer* phy, RADIOLIB_PIN_TYPE pin) {
 }
 
 int16_t AFSKClient::tone(uint16_t freq, bool autoStart) {
+  if(freq == 0) {
+    return(ERR_INVALID_FREQUENCY);
+  }
+
   if(autoStart) {
     int16_t state = _phy->transmitDirect();
     RADIOLIB_ASSERT(state);