Reference Request: Poincare-Hopf Index Theorem

Question

When I read Griffiths' Algebraic Curves page 23, it states Poincare-Hopf formula for differential forms that is smooth except finite isolated singularities. I tried to find reference for it, but googling and Gulliemin & Pollack's text did not help, because G&P's text does it for smooth vector fields (which can be turned into smooth 1-forms, but not smooth except finite isolated singularities) Do you know any reference for this type? Or do you know reference for Poincare-Hopf theorem for meromorphic forms, as it is the form that textbook uses? Thank you.