SPI hanging on trivial code

[AndyA]

I can't see what I'm doing wrong here, I get the feeling I'm missing something obvious.

Teensy4.1, Arduino IDE 2.0.4

#include <SPI.h>

int count;

void setup() {
  delay(1000);
  count = 0;
}

void loop() {
 Serial.printf("Loop %d\r\n",++count);
 SPI.beginTransaction(SPISettings(1000000, MSBFIRST, SPI_MODE0));
 if (count == 10)
   SPI.transfer(0xff);
 SPI.endTransaction();
 delay(1000);
}

Gives the result

Loop 1
Loop 2
Loop 3
Loop 4
Loop 5
Loop 6
Loop 7
Loop 8
Loop 9
Loop 10

and then locks up. No matter what I do the system dies as soon as I call SPI.transfer()

SPI1 and SPI2 show exactly the same results as SPI.

Please tell me what stupid mistake I'm making because everything I can find says this should work.

[PaulStoffregen]

Add SPI.begin() in your setup() function.

[AndyA]

Thank you.
Now excuse me while I go and kick myself a few more times...