Understanding the behaviour of waitpid(-1, NULL, WNOHANG)

As far as I understand from waitpid's man page, when calling waitpid with WNOHANG, the function should return 0 if there are no child processes to wait for.
When running the following program -

===
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#define N 5

void error_exit(char *str)
{
      perror(str);
      exit(1);
}

void sigchld_handler(int sig)
{
      pid_t      pid;
      do {
            pid = waitpid(-1, NULL, WNOHANG);
            printf("child %d\n",pid);
      } while ( pid > 0 );
      
      if ( errno == ECHILD )
            puts("errno=ECHILD");
      if ( pid < 0 )
            error_exit("waitpid");

      if ( signal(SIGCHLD, sigchld_handler) == SIG_ERR )
            error_exit("signal");
}

int main()
{
      int       i;
      pid_t       pid;
      
      if ( signal(SIGCHLD, sigchld_handler) == SIG_ERR )
            error_exit("signal");
      
      for ( i = 0; i < N; i += 1 ) {
            pid = fork();
            if ( pid < 0 )
                  // fork error
                  error_exit("fork");
            else if ( pid == 0 ) {
                  // child process - sleep
                  sleep(N-i+1);
                  exit(0);
            }
      }
      while(1);
      
      return 0;
}
===

I get this output -

===
child 5398
child 0
child 5397
child 0
child 5396
child 0
child 5395
child 0
child 5394
child -1
errno=ECHILD
waitpid: No child processes
===

As you can see, after all child processes have been "collected", waitpid returns -1 (which means an error occured) and sets errno to ECHILD. I'd like to know why do I get this error, and what is the problem with my code.

TIA

Edit:
My kernel version is 2.6.4 (also tried 2.4.22), and my GCC version is 3.3.3 (also tried 3.2.3).