# What do NaNs represent in an SPM β-map?

I've estimated a model for an fMRI dataset, so I now have a beta_*.nii file for each regressor in my model. In a seperate step, I've defined ROIs using marsbar and proceeded to extract the corresponding voxels in one of these β-maps.

However, the ROI extracted from said β-map contains only NaN values. Why should there be NaNs at all? What does such a null value represent in the context of model βs?