Shouldn't QChar::digitValue() return -1 instead of zero when invalid ucs4 character was passed?

Following code is used in QChar::digitValue(), I think that 'return 0' should be changed to 'return -1', because zero has meaning that the character is a digit value.

int QChar::digitValue(uint ucs4)
{
if (ucs4 > UNICODE_LAST_CODEPOINT)
return 0;
return qGetProp(ucs4)->digitValue;
}