The response of an interferometer becomes complicated for a gravitational wave shorter than the armlength of the detector, as the nature of the wave appears strongly. We study how the parameter estimation for merging massive-black-hole binaries is affected by this complicated effect in the case of LISA. It is shown that the three-dimensional positions of some binaries might be determined much more accurately than in past estimations that use the long wave approximation. For equal mass binaries this improvement is most prominent at similar to10(5) M..