diff --git a/libraries/WiFiS3/src/Modem.cpp b/libraries/WiFiS3/src/Modem.cpp
index 3ae38f335..249a3da7e 100644
--- a/libraries/WiFiS3/src/Modem.cpp
+++ b/libraries/WiFiS3/src/Modem.cpp
@@ -86,7 +86,7 @@ bool ModemClass::passthrough(const uint8_t *data, size_t size) {
 }
 
 /* -------------------------------------------------------------------------- */
-void ModemClass::write_nowait(const string &cmd, string &str, char * fmt, ...) {
+void ModemClass::write_nowait(const string &cmd, string &str, const char * fmt, ...) {
 /* -------------------------------------------------------------------------- */   
    memset(tx_buff,0x00,MAX_BUFF_SIZE);
    va_list va;
@@ -106,7 +106,7 @@ void ModemClass::write_nowait(const string &cmd, string &str, char * fmt, ...) {
 
 
 /* -------------------------------------------------------------------------- */
-bool ModemClass::write(const string &prompt, string &data_res, char * fmt, ...){
+bool ModemClass::write(const string &prompt, string &data_res, const char * fmt, ...){
 /* -------------------------------------------------------------------------- */  
    data_res.clear();
    memset(tx_buff,0x00,MAX_BUFF_SIZE);
diff --git a/libraries/WiFiS3/src/Modem.h b/libraries/WiFiS3/src/Modem.h
index 3d8aee5b5..78539e7e3 100644
--- a/libraries/WiFiS3/src/Modem.h
+++ b/libraries/WiFiS3/src/Modem.h
@@ -24,8 +24,8 @@ class ModemClass {
 
   void begin(int badurate = 115200);
   void end();
-  bool write(const std::string &cmd, std::string &str, char * fmt, ...);
-  void write_nowait(const std::string &cmd, std::string &str, char * fmt, ...);
+  bool write(const std::string &cmd, std::string &str, const char * fmt, ...);
+  void write_nowait(const std::string &cmd, std::string &str, const char * fmt, ...);
 
   bool passthrough(const uint8_t *data, size_t size);
   void avoid_trim_results() {