error: 'dtostrf' was not declared in this scope

[smart-fun]

Hi,
I try to use the library for a Adafruit Feather nrf52 projetct but get this error:

C:\Users\Arnaud\Documents\Arduino\libraries\MatrizLed\src\MatrizLed.cpp: In member function 'void MatrizLed::escribirCifra(int, int)':
C:\Users\Arnaud\Documents\Arduino\libraries\MatrizLed\src\MatrizLed.cpp:289:5: error: 'dtostrf' was not declared in this scope
  289 |     dtostrf(cifra, getDeviceCount(), 0, formateado);
      |     ^~~~~~~
exit status 1
Error compiling for board Adafruit Feather nRF52832.

could you help?

thanks!