  We present a no-go result for postselection measurements where the
conditional expectation value of a joint system-device observable under
postselection is nothing else than the conventional expectation value. Such a
no-go result relies on the rank-m degenerate of the joint observable, where m
is the dimension of the device subspace. Remarkable, we show that the error and
disturbance in quantum measurements obey the no-go result, which implies that
the error-disturbance uncertainty is unaffected under postselection
measurements.
