Although implicit guarantees are widely used in the shadow banking system, we know very little about its qualitative and quantitative properties. In this paper, we use a micro-level data set on China's shadow bank products to quantify the risk of implicit guarantees. We find a robust empirical fact that banks extend more implicit guarantees to their shadow bank debt (i.e., wealth management products) when their own default risks increase. Our result shows that this effect is particularly stronger when riskier banks plan to issue certificates of deposits in the interbank market. A simple model that is based on a signaling game is proposed to rationalize this fact. The key mechanism of the model is that as a bank's reputation becomes worse, it has stronger incentives to send positive signals to the market, i.e., to boost the realized returns of its shadow bank obligations, although it has no obligation to do so. Our findings show that implicit guarantees have nonlinear negative effects on bank fundamentals and the risk-weight of off-balance-sheet exposure should be increasing in banks' default risks.