First and foremost - one should read the documentation for the LROC PDS RDR products ( http://lroc.sese.asu.edu/data/LRO-L-LROC-5-RDR-V1.0/LROLRC_2001/DOCUMENT/RDRSIS.PDF) and when available, the README file for an individual product family (for the Eratosthenes DTM refer to http://lroc.sese.asu.edu/data/LRO-L-LROC-5-RDR-V1.0/LROLRC_2001/DATA/SDP/NAC_DTM/ERATOSTHNS1/NAC_DTM_ERATOSTHNS1_README.TXT), before attempting to use any of the data products.
In your particular case, you are attempting to read elevation data from the ortho-rectified image, not from the actual DTM product. For each NAC DTM product the LROC team generates, there are numerous files created: DTM, ortho-rectified images, slope map, confidence map. If you download and ingest the actual Eratosthenes DTm, you should be able to read the elevation values.
It can not be stressed enough, please read the documentation first, which should preclude false starts such as this.