STM32F103 Timer IRQ handler never executes - timer

Question

I'm trying to setup a timer that executes every 1ms, in order to estimate the execution time of a function. I attach the code at the end of the post. I'm pretty sure about the configuration of the timer, since I have done only very little modification to a working demo (iNemo_AHRS_continous demo, even the function for prvFindFactors is taken by such a demo).
Unfortunately, in this case my ISR is never executed, any idea of the reason? Thanks
void iNemoProfilerConfig(void)
{
unsigned short a;
unsigned short b;
unsigned long n;
/* This value is the frequency interrupts in Hz */
unsigned short frequency = 1000;
TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
NVIC_InitTypeDef NVIC_InitStructure;
/* Enable timer clocks */
RCC_APB1PeriphClockCmd( RCC_APB1Periph_TIM4, ENABLE );
TIM_ClearITPendingBit(TIM4, TIM_IT_Update);
TIM_TimeBaseStructInit( &TIM_TimeBaseStructure );
/* Time base configuration for timer 2 - which generates the interrupts. */
n = SystemCoreClock/frequency;
prvFindFactors( n, &a, &b );
TIM_TimeBaseStructure.TIM_Period = b - 1;
TIM_TimeBaseStructure.TIM_Prescaler = a - 1;
TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1;
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
TIM_TimeBaseInit( TIM4, &TIM_TimeBaseStructure );
TIM_ARRPreloadConfig( TIM4, ENABLE );
NVIC_InitStructure.NVIC_IRQChannel = TIM4_IRQn;
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 13;
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
NVIC_Init( &NVIC_InitStructure );
}
void Enable_Timer4(FunctionalState command) {
TIM_ClearITPendingBit(TIM4, TIM_IT_Update);
TIM_ITConfig( TIM4, TIM_IT_Update, command );
}
void TIM4_IRQHandler(void)
{
USART1_Printf("INTERRUPT = %i \r\n", counter);
if(TIM_GetITStatus(TIM4, TIM_IT_Update))
{
//xTim2Raised=SET;
counter++;
/* Clear the IRQ bit */
TIM_ClearITPendingBit(TIM4, TIM_IT_Update);
}
}
//main chunk
int main(void)
{
SystemInit();
/* At this stage the microcontroller clock setting is already configured,
this is done through SystemInit() function which is called from startup
file (startup_stm32f10x_xx.s) before to branch to application main.
To reconfigure the default setting of SystemInit() function, refer to
system_stm32f10x.c file
*/
USART1_Init();
for(uint32_t i=0; i<0xFFFFF;i++);
iNemoTimerConfig();
iNemoProfilerConfig();
iNEMO_AHRS_Start();
LSM_Start();
L3G_Start();
/* Wait some seconds in order to ensure the user opens the VCOM terminal */
for(uint32_t i=0;i<0x1FFFFFF;i++);
Enable_Timer(ENABLE);
Enable_Timer4(ENABLE);
while(1)
{
counter = 0;
iNEMO_AHRS_Attitude();
USART1_Printf("counter = %i \r\n", counter);