Increment and Decrement Implementation Not Working Correctly

Increment and decrement don't appear to be functioning correctly. I did the following

$m->set('testkey',0);
var_dump($m->get('testkey'));
var_dump($m->increment('testkey'));
var_dump($m->get('testkey'));

which output

int(0) 
bool(true) 
int(2147483647)

2147483647 is 0x7FFFFFFF

I was expecting the value returned to be 1.
