Why can't i send a specific number with i2c?

I have started writing an i2c driver for the stm32f051c8 and have stumbled upon an issue.

The code i use to test a very basic version is:

	RCC->AHBENR|= 1<<18; // gpio clock enable gpiob
	RCC->APB1ENR|= 1<<21; // enable i2c1 clock
	GPIOB->MODER|= 2<<12; // alternate function for two lines
	GPIOB->MODER|= 2<<14;
	GPIOB->AFR[0]|= (1<<24) | (1<<28); // i2c for alternate function
	I2C1->TIMINGR= 0x10420F13; // taken from the datasheet
	
	I2C1->CR1|= 1<<0; // i2c enable
	I2C1->CR2|= (1<<25) | (1<<16); // 1 byte, autoend enable 
	I2C1->CR2|= 0x50<<1; // slave address
	I2C1->CR2|= 1<<13; // start 
	

When i choose the slave address 0x59, for example, i do get the correct output (pic), but when i leave it 0x50, nothing comes out from either line.

thanks