Incompatible pointer types error when assigning between signed/unsigned char array to unsigned/signed char *

[zu2]

  signed char	a[100];
unsigned char	b[100];

int main(int argc, char **argv)
{
	unsigned char *uc=a;
	signed   char *sc=b;
}

0540-ptrincdec.c(6): Error: Incompatible pointer types at 'a'
0540-ptrincdec.c(7): Error: Incompatible pointer types at 'b'

[zu2]

It is not allowed by the standard, but should it be limited to a warning?