RX interrupt using mBed OS Serial throwing Mutex error at runtime

Question

Running mBed OS on a B-L475E-IOT01A1 board but currently not using any threads or Mutexs. I borrowed an example test script to isolate the bit of code not working, shown below. This just reads and stores the buffer.

#include "mbed.h"

Serial device(USBTX, USBRX);  // tx, rx

DigitalOut led1(LED1);

void Rx_interrupt();

// Circular buffers for serial TX and RX data - used by interrupt routines
const int buffer_size = 255;
// might need to increase buffer size for high baud rates
char rx_buffer[buffer_size+1];
// Circular buffer pointers
// volatile makes read-modify-write atomic 
volatile int rx_in=0;
volatile int rx_out=0;

// main test program
int main() {
    device.baud(9600);

// Setup a serial interrupt function to receive data
    device.attach(&Rx_interrupt, Serial::RxIrq);

    while (1) {

    }
}

// Interupt Routine to read in data from serial port
void Rx_interrupt() {
    led1=1;
// Loop just in case more than one character is in UART's receive FIFO buffer
// Stop if buffer full
    while ((device.readable()) && (((rx_in + 1) % buffer_size) != rx_out)) {
        rx_buffer[rx_in] = device.getc();
        rx_in = (rx_in + 1) % buffer_size;
    }
    led1=0;
    return;
}

This script does essentially nothing. Yet every time I transmit data to it to trigger the interrupt the terminal prints the following.

++ MbedOS Error Info ++
Error Status: 0x80010133 Code: 307 Module: 1
Error Message: Mutex: 0x1000158C, Not allowed in ISR context
Location: 0x800A64D
Error Value: 0x1000158C
Current Thread: main  Id: 0x20000354 Entry: 0x8007BF5 StackSize: 0x1000 StackMem: 0x10000418 SP: 0x20017EA8 
For more info, visit: https://mbed.com/s/error?error=0x80010133&tgt=DISCO_L475VG_IOT01A
-- MbedOS Error Info --

Even if I delete the content of the Rx_Interrupt function so it just calls an empty function and immediately returns it still throws the error. I'm a bit rusty on coding but can't for the life of me work out why. Is the use of the serial attach function fully disabled in mBed OS because of threading?

UPDATE: I updated my version of mBed to the newest version and rewrote using the unbuffered serial class and now it works fine. Still not certain why the old mBed OS version didn't work but this at least solves the issue