This study provides the first empirical assessment of the causal impact of bureaucratic corruption on firms' financial constraints in Nigeria by calculating treatment effects using linear and non-linear estimators to account for potential heterogeneous treatment effects across firm groups. Formally, the theoretical framework models how corruption may facilitate or restrain firms' financial access by shaping their cost functions, which consequently influences their success or failure and ability to raise the collateral for borrowing. My analysis, using the bivariate probit method and two binary instruments, reveals that corruption significantly increases the probability of a representative MSME and firm being financially constrained by approximately 62 to 64 and 61 to 63 percentage points, respectively. When the IV estimator is utilized to calculate local effects, I find that the effect is about 90 to 91 percentage points for a typical MSME facing obstacles with obtaining business licenses and permits and tax administration, respectively. The effect is 92 percentage points for all firms using both instruments. Furthermore, the results show that Nigerian MSMEs are about 17 to 19 percentage points more likely to be financially constrained than large firms and that corruption's impact on firms' access to finance does not depend on firm size. Finally, firms that perceive corruption as a ''minor" barrier experience the most difficulty obtaining external finance. This study highlights the severe constraint that corruption poses to Nigerian firms' access to finance and advocates for regulatory amendments to address issues with the tax administration and the ease of obtaining business licenses and permits.