Help Somebody

While playing Sandbox I saved my model then it gave me this code:
Step() failed!: [string "--File:
.\scripts\shared\functional\SANDBOX..."]:33: attempt to index field'ChassisMaterial' (a nil value)
If anybody can help please I want to continue to build my cars
Step() failed!: [string "--File:
.\scripts\shared\functional\SANDBOX..."]:33: attempt to index field'ChassisMaterial' (a nil value)
If anybody can help please I want to continue to build my cars
