Protothread call on __LINE__ greater than 256: error: case label value exceeds

Question

I use protothreads on my embedded device (STM32, GCC) as it's lightweight, and saves me to handle a complex state machine by myself.

As my main program grows, a PT_* function is called on a line number greater than 256

struct pt main_pt;
int main(void)
{
    PT_BEGIN(pt);
    while(true)
    {
        /*... 256 lines further*/
        PT_WAIT_UNTIL(pt, flag);
        PT_YIELD(pt);
    }
    PT_END(pt);
}

And at compile, it throws this error:

main.c: In function 'main':    
main.c:256:4: error: case label value exceeds maximum value for type [-Werror]
    PT_WAIT_UNTIL(pt, flag);
main.c:257:4: error: case label value exceeds maximum value for type [-Werror]
    PT_YIELD(pt);

If PT_YIELD or PT_WAIT_UNTIL is placed on a line number lower than 256 it does compile.

Here is snippet for PT_YIELD, PT_WAIT_UNTIL, LC_SET and struct pt:

typedef uint32_t lc_t;
struct pt {
  lc_t lc;
};
#define LC_RESUME(s) switch(s) { case 0:
#define LC_SET(s) s = __LINE__; case __LINE__:
#define PT_BEGIN(pt) { unsigned int PT_YIELD_FLAG = 1; LC_RESUME((pt)->lc)
#define PT_WAIT_UNTIL(pt, condition)            \
  do {                      \
    LC_SET((pt)->lc);               \
    if(!(condition)) {              \
      return PT_WAITING;            \
    }                       \
  } while(0)
#define PT_YIELD(pt)                \
  do {                      \
    PT_YIELD_FLAG = 0;              \
    LC_SET((pt)->lc);               \
    if(PT_YIELD_FLAG == 0) {            \
      return PT_YIELDED;            \
    }                       \
  } while(0)

Why can't "case __LINE__:" exceed 256 as it's store on uint32_t?