Add error message when conversion of EEG locs to head space fails (#1…

…1080)

doc/changes/latest.inc

@@ -44,8 +44,10 @@ Bugs
 - Fix bug in :class:`mne.viz.Brain` constructor where the first argument was named ``subject_id`` instead of ``subject`` (:gh:`11049` by `Eric Larson`_)
 - Fix bug in :ref:`mne coreg` where the MEG helmet position was not updated during ICP fitting (:gh:`11084` by `Eric Larson`_)
 - Document ``height`` and ``weight`` keys of  ``subject_info`` entry in :class:`mne.Info` (:gh:`11019` by :newcontrib:`Sena Er`)
+- Fixed bug in :func:`mne.viz.plot_filter` when plotting filters created using ``output='ba'`` mode with ``compensation`` turned on. (by `Marian Dovgialo`_)
 - Fix bug in :func:`mne.viz.plot_filter` when plotting filters created using ``output='ba'`` mode with ``compensation`` turned on. (:gh:`11040` by `Marian Dovgialo`_)
 - Fix bug in :func:`mne.viz.plot_topomap` when providing ``sphere="eeglab"`` (:gh:`11081` by `Mathieu Scheltienne`_)
+- Applying a montage where EEG locations are not in head space (or unknown space) without fiducials will now raise an error message. (:gh:`11080` by `Marijn van Vliet`_)
 
 API changes
 ~~~~~~~~~~~

mne/channels/montage.py

@@ -668,11 +668,19 @@ def transform_to_head(montage):
     # Get fiducial points and their coord_frame
     native_head_t = compute_native_head_t(montage)
     montage = montage.copy()  # to avoid inplace modification
+
     if native_head_t['from'] != FIFF.FIFFV_COORD_HEAD:
         for d in montage.dig:
             if d['coord_frame'] == native_head_t['from']:
                 d['r'] = apply_trans(native_head_t, d['r'])
                 d['coord_frame'] = FIFF.FIFFV_COORD_HEAD
+            elif d['kind'] == FIFF.FIFFV_POINT_EEG:
+                raise RuntimeError(
+                    f'Could not transform EEG channel {d["ident"]} position '
+                    f'from {_verbose_frames[d["coord_frame"]]} to head '
+                    'coordinates. Fiducial points are either missing or '
+                    'specified in a different coordinate frame than the EEG '
+                    'channel locations.')
     return montage
 
 

mne/channels/tests/test_montage.py

@@ -1210,6 +1210,17 @@ def test_transform_to_head_and_compute_dev_head_t():
             montage_polhemus
         ))
 
+    # Test errors when transforming without fiducials explicitly where points
+    # are tagged to be not in head or unknown coord space.
+    montage_without_fids = make_dig_montage(
+        ch_pos={"ch_1": np.array([1, 2, 3]),
+                "ch_2": np.array([4, 5, 6]),
+                "ch_3": np.array([7, 8, 9])},
+        coord_frame="mri")  # MRI coordinate space
+    with pytest.raises(RuntimeError, match='Could not transform EEG channel'):
+        with pytest.warns(RuntimeWarning, match='Fiducial point .* not found'):
+            transform_to_head(montage_without_fids)
+
 
 def test_set_montage_with_mismatching_ch_names():
     """Test setting a DigMontage with mismatching ch_names."""

mne/io/_digitization.py

@@ -300,7 +300,7 @@ def _get_fid_coords(dig, raise_error=True):
     if len(fid_coord_frames) > 0 and raise_error:
         if set(fid_coord_frames.keys()) != set(['nasion', 'lpa', 'rpa']):
             raise ValueError("Some fiducial points are missing (got %s)." %
-                             fid_coords.keys())
+                             fid_coord_frames.keys())
 
         if len(set(fid_coord_frames.values())) > 1:
             raise ValueError(