Why is it ambiguous to call overloaded ambig(long) and ambig(unsigned long) with an integer literal?

When compiling

void ambig(  signed long) { }
void ambig(unsigned long) { }

int main(void) { ambig(-1); return 0; }

I get

error C2668: 'ambig' : ambiguous call to overloaded function
    could be 'void ambig(unsigned long)'
    or 'void ambig(long)'
while trying to match the argument list '(int)'

I know I can 'fix' it by saying -1L instead of -1, but why/how exactly is this considered ambiguous in the first place?