iOS NSUnknownKeyException

请问一下出现这个问题是什么原因：

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<NSCFString 0x1742792c0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key asBuilder.'
First throw call stack:
(0x1824682d8 0x193dc40e4 0x182467f5c 0x183333bdc 0x183280db4 0x1000d1dc0 0x1001508f8 0x10015065c 0x183369f9c 0x182420240 0x18241f4e4 0x18241d594 0x1823492d4 0x18bc4f6fc 0x186f46f40 0x1001135dc 0x19446ea08)
libc++abi.dylib: terminating with uncaught exception of type NSException