The presence of a BRST symmetry in topologically twisted gauge theories makes a precise analysis of these theories feasible. While the global BRST symmetry suggests that correlation functions of BRST exact observables vanish, this decoupling might be obstructed due to a contribution from the boundary of field space. Motivated by divergent BRST exact observables on the Coulomb branch of Donaldson-Witten theory, we put forward a new prescription for the renormalization of correlation functions on the Coulomb branch. This renormalization is based on the relation between Coulomb branch integrals and integrals over a modular fundamental domain, and establishes that BRST exact observables indeed decouple in Donaldson-Witten theory. arXiv:1901.03540v2 [hep-th] 9 Aug 2019 5. Evaluation of correlation functions of Q-exact observables 30 6. Discussion and conclusion 31 A. Modular forms and theta functions 33 B. Siegel-Narain theta function 34 C. The self-dual twisted operator 35 -1 -and whose restriction to H 2 (M, Z)×H 2 (M, Z) is an integral bilinear form with signature (1, b 2 − 1). The bilinear form provides the quadratic form Q(k) := B(k, k) ≡ k 2 , which can be brought to a simple standard form [15, Section 1.1.3]. We denote the period point by J, i.e. the harmonic two-form, satisfying * J = J ∈ H 2 (M, R), J 2 = 1, (3.2)