Overcollateralization refers to the process of posting more collateral than is needed to obtain or secure financing. It represents a form of internal credit enhancement because the additional collateral can be used to absorb losses. For example, if at issuance the principal amount of a bond issue is $100 million and the value of the collateral is $110 million, the amount of overcollateralization is $10 million. Over time, the amount of overcollateralization changes, for instance as a result of amortization, prepayments or defaults in the case of MBS. A major problem associated with overcollateralization is the valuation of the collateral. For example, one of the most significant contributors to the 2007–2009 credit crisis was a valuation problem with the residential housing assets backing MBS. Many properties were originally valued in excess of the worth of the issued securities. But as property prices fell and homeowners started to default on their mortgages, the credit quality of many MBS declined sharply. The result was a rapid rise in yields and panic among investors in these securities.