Update gammapy/datasets/map.py
@@ -789,7 +789,7 @@ class MapDataset(Dataset):
if self.mask_fit and other.mask_fit:
self.mask_fit.stack(other.mask_fit)
elif other.mask_fit:
- self.mask_fit = other.mask_fit
+ self.mask_fit = other.mask_fit.copy()
if self.gti and other.gti:
self.gti.stack(other.gti)
