Using the China's mandate in 2014 requiring private firms to publicly disclose their annual reports, we examine how more public information about private firms affect public firms disclosure of major customer identities. Our difference-in-difference analysis reveals that the mandate negatively affects public firms tendency to disclose their customers identities. We infer from this finding that the mandate increases proprietary costs of firms revealing the identities of customers with more publicly available information. Further support for this increased proprietary cost inference comes from evidence that that the negative effect is more pronounced for firms more concerned about losing their customers. We also find that the negative effect is less pronounced for firms relying more on external financing, consistent with there being a trade-off between proprietary costs and capital market benefits of disclosing the identities of customers. Overall, our study provides novel insights on how the regulation on private firm disclosure can have unintended effects on public firms disclosure practices.