diff --git a/mne/channels/channels.py b/mne/channels/channels.py
index 2a772e77491..aec1891d5b6 100644
--- a/mne/channels/channels.py
+++ b/mne/channels/channels.py
@@ -286,6 +286,10 @@ def set_channel_types(self, mapping):
             _check_set(self.info['chs'][c_ind], self.info['projs'], ch_type)
             unit_old = self.info['chs'][c_ind]['unit']
             unit_new = _human2unit[ch_type]
+            if unit_old not in _unit2human:
+                raise ValueError("Channel '%s' has unknown unit (%s). Please "
+                                 "fix the measurement info of your data."
+                                 % (ch_name, unit_old))
             if unit_old != _human2unit[ch_type]:
                 warn("The unit for channel %s has changed from %s to %s."
                      % (ch_name, _unit2human[unit_old], _unit2human[unit_new]))
diff --git a/mne/channels/tests/test_channels.py b/mne/channels/tests/test_channels.py
index 32649951deb..660d15204da 100644
--- a/mne/channels/tests/test_channels.py
+++ b/mne/channels/tests/test_channels.py
@@ -106,6 +106,11 @@ def test_set_channel_types():
         assert_true(info['chs'][idx]['unit'] == FIFF.FIFF_UNIT_V)
         assert_true(info['chs'][idx]['coil_type'] == FIFF.FIFFV_COIL_EEG)
 
+    # Test meaningful error when setting channel type with unknown unit
+    raw.info['chs'][0]['unit'] = 0.
+    ch_types = {raw.ch_names[0]: 'misc'}
+    assert_raises(ValueError, raw.set_channel_types, ch_types)
+
 
 def test_read_ch_connectivity():
     "Test reading channel connectivity templates"