Handle cases with no fnirs picks for interpolation

mne/channels/interpolation.py

@@ -233,13 +233,16 @@ def _interpolate_bads_nirs(inst, method='nearest', verbose=None):
     from mne.preprocessing.nirs import _channel_frequencies,\
         _check_channels_ordered
 
-    freqs = np.unique(_channel_frequencies(inst))
     # Returns pick of all nirs and ensures channels are correctly ordered
+    freqs = np.unique(_channel_frequencies(inst))
     picks_nirs = _check_channels_ordered(inst, freqs)
+    if len(picks_nirs) == 0:
+        return
+
     nirs_ch_names = [inst.info['ch_names'][p] for p in picks_nirs]
     bads_nirs = [ch for ch in inst.info['bads'] if ch in nirs_ch_names]
 
-    # select the bad meg channel to be interpolated
+    # select the bad nirs channel to be interpolated
     if len(bads_nirs) == 0:
         picks_bad = []
     else:

mne/preprocessing/nirs/nirs.py

@@ -58,7 +58,7 @@ def short_channels(info, threshold=0.01):
 
 def _channel_frequencies(raw):
     """Return the light frequency for each channel."""
-    picks = _picks_to_idx(raw.info, 'fnirs', exclude=[])
+    picks = _picks_to_idx(raw.info, 'fnirs', exclude=[], allow_empty=True)
     freqs = np.empty(picks.size, int)
     for ii in picks:
         freqs[ii] = raw.info['chs'][ii]['loc'][9]
@@ -69,7 +69,7 @@ def _check_channels_ordered(raw, freqs):
     """Check channels followed expected fNIRS format."""
     # Every second channel should be same SD pair
     # and have the specified light frequencies.
-    picks = _picks_to_idx(raw.info, 'fnirs', exclude=[])
+    picks = _picks_to_idx(raw.info, 'fnirs', exclude=[], allow_empty=True)
     if len(picks) % 2 != 0:
         raise ValueError(
             'NIRS channels not ordered correctly. An even number of NIRS '