sigprocmask with SIG_BLOCK doesn't block SIGALRM

Question

Its a sample program to check if sigprocmask actually blocks the SIGALRM signal associated with my timer. Turns out it doesn't. My handler gets called even when the signal is BLOCKED. Would really appreciate if someone can point out why that is. Here is the code:

#include <signal.h> 
#include <time.h>
#include <stdio.h>
#include <stdlib.h>

timer_t timer_ID;
static void timer_handler(int signo, siginfo_t *sig_info, void *sig_ucontext){  
    if(sig_info->si_value.sival_ptr != &timer_ID){
        printf("This signal is stray and didnt come from the timer we setup. Exiting the program!\n");
        exit(1);
     }
     else{
        printf("Caught signal %d from timer\n", signo);
     }
     return;
}//func end

void main(void)
{   
    struct sigevent sev;         // for creating the timer  
    sigset_t sigmask;   
    struct sigaction sa = {0};          
    struct itimerspec its;  
    its.it_interval.tv_sec = 0;
    its.it_interval.tv_nsec = 500000; 

    its.it_value.tv_sec = its.it_interval.tv_sec;
    its.it_value.tv_nsec = its.it_interval.tv_nsec;

    sa.sa_flags = SA_SIGINFO | SA_RESTART; 
    sa.sa_sigaction = timer_handler;

    if (sigaction(SIGALRM, &sa, NULL) == -1){
       fprintf(stderr, "sigaction register error\n");
       exit(1);
    }   
    //BLOCK SIG
    sigemptyset (&sigmask);
    sigaddset (&sigmask, SIGALRM);
    if (sigprocmask(SIG_BLOCK, &sigmask, NULL) == -1) {
        fprintf (stderr, "Failed to set signal mask!\n");
        exit (1);
    }   

    //CREATE TIMER
    sev.sigev_signo = SIGALRM; 
    sev.sigev_notify = SIGEV_SIGNAL; 
    sev.sigev_value.sival_ptr = &timer_ID; 

    if(timer_create(CLOCK_MONOTONIC, &sev, &timer_ID) != 0){ 
        printf("Create timer error!\n");        
        exit (1);
    } 
    // START TIMER
    if(timer_settime(timer_ID, 0, &its, NULL) !=0){
        printf("ERROR setting the timer!!!!\n");
        exit(1);
    }

    while(1);  //signal is blocked so should never go to handler
}//main end