During the last years the electron beams produced by Laser-Plasma Accelerators were used for the first time to generate synchrotron radiation in an undulator. Both the emittance and especially the energy spread of a laser plasma accelerator beam are significantly larger than the emittance and the energy spread of beams from conventional accelerators. Beams with a high energy spread produce in a conventional undulator photon beams with a high wavelength spread. In this paper a novel modified undulator design is presented which can reduce the energy spread of the photon beam caused by the energy spread of the electron beam. The magnetic concept is analysed in detail by tracking test electrons through the device. An experiment to test this concept is in preparation.