The time-evolution of a wave packet in a perfectly periodic lattice potential is considered for an electron that is in a zero-field Bloch energy eigenstate prior to the adiabatic application of a uniform d.c. electric field. Power series expansions in time are constructed for both the exact wave function and the Houston wave function. A comparison of the coefficients in these two expansions shows that the Houston function diverges instantly and drastically from the exact wave function.A special-case example is presented for which the exact wave function is pure-real for all values of time whereas the corresponding Houston function is not.