How can I fill in "undefined values" in a dfs2 bathymetry file?

Hi all, I’m working on a MIKE BW model and need to prepare a bathymetry file. I created an .asc file from my DEM data and converted it to a dfs2 file using the Grd2Mike tool in the Toolbox. However, the dfs2 file shows "undefined values" over land areas as shown in the figure. Instead of manually filling these gaps (which is time-consuming and prone to errors), is there a more efficient way to handle this?