Sec. 3402. Income Tax Collected at Source

Section 3402. Income tax collected at source
(a) Requirement of withholding. (1) In general. Except as otherwise provided in this section, every employer making payment of wages shall deduct and withhold upon such wages a tax determined in accordance with tables or computational procedures prescribed by the Secretary....
26 USC 3402

Do you see where in the very first line it says "Except as otherwise provided". Well, what about Subsection (n):

(n) Employees incurring no income tax liability Not withstanding any other provisions of this section an employer shall not be required to deduct and withhold any tax under this chapter upon a payment of wages to an employee if there is in effect with respect to such payment a withholding exemption certificate furnished to the employer by the employee certifying that the employee -

(1) incurred no liability for income tax imposed under subtitle A for
      his preceding taxable year, and
(2)anticipates that he will incur no liability for income tax imposed
     under subtitle A for his current taxable year.....

So, if a person certifies that they have no liability for tax under Subtitle A, then their employer does not have to withhold any income tax from their pay. So, the question is: do you have any liability for tax under subtitle A? Well then, if not, does this next section apply to your situation?

(p) Voluntary withholding agreements. The Secretary is authorized by regulations to provide for withholding -

(1) from remuneration for services performed by an employee for his
      employer which does not constitute wages, and
(2) from any other type of payment with respect to which the Secretary
      finds that withholding would be appropriate under the provisions of this chapter,

if the employer and the employee, or in the case of any other type of payment the person making and the person receiving the payment, agree to such withholding. Such agreement shall be made in such form and manner as the Secretary may by regulations provide. For purposes of this chapter (and so much of subtitle F as relates to this chapter) remuneration or other payments with respect to which such agreement is made shall be treated as if they were wages paid by an employer to an employee to the extent such remuneration is paid or other payments are made during the period for which the agreement is in effect .....(emphasis added)
26 USC 3402