Most of the existing reverse mortgage products include a guarantee that the loan value cannot exceed the house value when the contract ends, which is called the No-Negative-Equity-Guarantee (NNEG). Although different models have been developed to price the NNEG, model risk is typically not investigated. We evaluate the cash flows of different reverse mortgage designs and the implied value of the embedded NNEG, taking into account model risk. We compare the values of the NNEG and their sensitivities to different parameters generated from two popular models in the literature for the house price process, i.e., the Geometric Brownian Motion model and the VAR Model. We calibrate the models using prices of regular mortgages and determine the corresponding price ranges for reverse mortgages. We find quite substantial price ranges for both models, indicating that there is substantial model risk in pricing the NNEG in reverse mortgage products. The degree of model risk is larger in the VAR model than in the GBM Model. We also find that, among the different parameters that can affect the value of the NNEG, it is most sensitive to the parameter that reflects the probability distribution of the future house price, which is the house price increase rate in the VAR model and the dividend rate in the GBM Model.