diff --git a/src/protocols/FSK4/FSK4.cpp b/src/protocols/FSK4/FSK4.cpp
index 4816513e..4eb9b7bf 100644
--- a/src/protocols/FSK4/FSK4.cpp
+++ b/src/protocols/FSK4/FSK4.cpp
@@ -54,7 +54,7 @@ int16_t FSK4Client::setCorrection(int16_t offsets[], float length) {
   return(RADIOLIB_ERR_NONE);
 }
 
-size_t FSK4Client::write(uint8_t* buff, size_t len) {
+size_t FSK4Client::write(const uint8_t* buff, size_t len) {
   size_t n = 0;
   for(size_t i = 0; i < len; i++) {
     n += FSK4Client::write(buff[i]);
diff --git a/src/protocols/FSK4/FSK4.h b/src/protocols/FSK4/FSK4.h
index 31df4101..757e556d 100644
--- a/src/protocols/FSK4/FSK4.h
+++ b/src/protocols/FSK4/FSK4.h
@@ -59,7 +59,7 @@ class FSK4Client {
       \param len Number of bytes to transmit.
       \returns Number of transmitted bytes.
     */
-    size_t write(uint8_t* buff, size_t len);
+    size_t write(const uint8_t* buff, size_t len);
 
     /*!
       \brief Transmit a single byte.