diff --git a/DMRSlot.cpp b/DMRSlot.cpp
index a296574..7db9c53 100644
--- a/DMRSlot.cpp
+++ b/DMRSlot.cpp
@@ -562,6 +562,9 @@ bool CDMRSlot::writeModem(unsigned char *data, unsigned int len)
 		if (m_rfState == RS_RF_AUDIO) {
 			m_rfN = data[1U] & 0x0FU;
 
+			if(m_rfN > 5U)
+				return false;
+
 			unsigned int errors = 0U;
 			unsigned char fid = m_rfLC->getFID();
 			if (fid == FID_ETSI || fid == FID_DMRA) {
@@ -802,6 +805,9 @@ bool CDMRSlot::writeModem(unsigned char *data, unsigned int len)
 
 				m_rfN = data[1U] & 0x0FU;
 
+				if(m_rfN > 5U)
+					return false;
+
 				// Regenerate the EMB
 				emb.getData(data + 2U);