ERDAS Imagine - recoding negative values to zero in an image

hello, i am working with an Aster DEM (.img file) in ERDAS Image. So areas of the DEM have negative values. I have since determined that these values are not valid, so i want to get them out of the image. I would prefer to recode them to 0. However, the recode function in ERDAS does not allow me to do this. Any ideas what i might be doing wrong?