diff --git a/src/nifreeze/model/base.py b/src/nifreeze/model/base.py
index 1759164..bcdca3e 100644
--- a/src/nifreeze/model/base.py
+++ b/src/nifreeze/model/base.py
@@ -22,6 +22,8 @@
 #
 """Base infrastructure for nifreeze's models."""
 
+from warnings import warn
+
 import numpy as np
 
 from nifreeze.exceptions import ModelNotFittedError
@@ -93,6 +95,9 @@ def __init__(self, mask=None, **kwargs):
         self._models = None  # For parallel (chunked) execution
 
         # Setup brain mask
+        if mask is None:
+            warn("No mask provided; consider using a mask to avoid issues in model optimization.")
+
         self._mask = mask
 
         self._datashape = None
diff --git a/src/nifreeze/registration/ants.py b/src/nifreeze/registration/ants.py
index f01978d..5b700da 100644
--- a/src/nifreeze/registration/ants.py
+++ b/src/nifreeze/registration/ants.py
@@ -382,12 +382,16 @@ def generate_command(
         settings["fixed_image_masks"] = [
             str(p) for p in _massage_mask_path(fixedmask_path, nlevels)
         ]
+    else:
+        warn("No fixed image mask provided; consider using a mask to avoid optimization issues.")
 
     # Set moving masks if provided
     if movingmask_path is not None:
         settings["moving_image_masks"] = [
             str(p) for p in _massage_mask_path(movingmask_path, nlevels)
         ]
+    else:
+        warn("No moving image mask provided; consider using a mask to avoid optimization issues.")
 
     # Set initalizing affine if provided
     if init_affine is not None: