Nominal spread of a non-treasury bond can be defined as the difference between the bond's yield and the yield to maturity of a benchmark treasury coupon security. For example, let us consider a 10-year bond. Assume that the yield to maturity of the Treasury bond is 7% and that of the non-treasury bond is 8.5%. Then, the normal spread would be 150 basis points (8.5% - 7%).
The two main drawbacks of this Spread are:
The term structure of the spot rates are not considered in both the bonds.
In case of callable and/or putable bonds, the cash flows of the non-treasury bonds may be altered due to the expected interest rate volatility.