No SDL event for space keypress if down+right or up+left already pressed

I am experimenting movements in a 2D game in SDL, and it works fine. However when I add the possibility for the character to shoot by pressing SPACE, I face a strange bug: If down and right keys or up and left keys are already pressed, no event is created when pressing SPACE. In other cases, an event is created, as it should be.

Here is the loop I use to get some events:

while (on == 1)
{
    SDL_PollEvent(&event);
    switch (event.type)
    {
        case SDL_KEYDOWN:
            switch (event.key.keysym.sym)
            {
                case SDLK_ESCAPE:
                    on = 0;
                    break;
                case SDLK_UP:
                    shooter1.dir[0] = 1; // shooter1 is an instance of a structure that
                                         // contains a position and an array representing
                                         // 4 directions (down, up, left, right)
                    break;
                case SDLK_DOWN:
                    shooter1.dir[1] = 1;
                    break;
                case SDLK_LEFT:
                    shooter1.dir[2] = 1;
                    break;
                case SDLK_RIGHT:
                    shooter1.dir[3] = 1;
                    break;
                case SDLK_SPACE:         // Eventually, the program does not go in this
                                         // case, even though the SPACE key is pressed
                    fprintf(stderr, "SPACE PRESSED\n");
                    break;
                default:
                    break;
            }
            break;
        case SDL_KEYUP:
            switch (event.key.keysym.sym)
            {
                case SDLK_UP:
                    shooter1.dir[0] = 0;
                    break;
                case SDLK_DOWN:
                    shooter1.dir[1] = 0;
                    break;
                case SDLK_LEFT:
                    shooter1.dir[2] = 0;
                    break;
                case SDLK_RIGHT:
                    shooter1.dir[3] = 0;
                    break;
                default:
                    break;
            }
            break;
        default:
            break;
    }
    event.type = 0;
    get_new_positions(&shooter1);
    SDL_BlitSurface(background, NULL, screen, &background_pos);
    blit_shooter(screen, &shooter1);
    SDL_Flip(screen);
}

Any idea of what this behavior is due to?