Raster to ASCII returns -9999 values

Hello!

I am trying to export the elevation from a DEM raster to an .asc file. I am using the ArcToolbox --> Conversion Tools --> Raster to ASCII function. The export is working, but if I open the output .asc with a text editor, it only contains -9999 (No Data). What could be the problem?

Regards,
Stefanie