diff --git a/src/ctapipe_io_lst/__init__.py b/src/ctapipe_io_lst/__init__.py
index 9a7113ed..0b1e05d0 100644
--- a/src/ctapipe_io_lst/__init__.py
+++ b/src/ctapipe_io_lst/__init__.py
@@ -686,6 +686,12 @@ def fill_trigger_info(self, array_event):
             )
 
         trigger.event_type = self._event_type_from_trigger_bits(trigger_bits)
+
+        if trigger.event_type == EventType.FLATFIELD:
+            waveform = array_event.r1.tel[tel_id].waveform
+            if waveform is not None and waveform.ndim == 2:
+                self.log.warning(f'Event {array_event.index.event_id} tagged as FLATFIELD, but has only one gain!')
+
         if trigger.event_type == EventType.UNKNOWN:
             self.log.warning(f'Event {array_event.index.event_id} has unknown event type, trigger: {trigger_bits:08b}')
 
@@ -702,8 +708,7 @@ def tag_flatfield_events(self, array_event):
         '''
         tel_id = self.tel_id
         waveform = array_event.r1.tel[tel_id].waveform
-
-        # needs to work for gain already selected or not
+        
         if waveform.ndim == 3:
             image = waveform[HIGH_GAIN].sum(axis=1)
         else:
@@ -713,16 +718,25 @@ def tag_flatfield_events(self, array_event):
         n_in_range = np.count_nonzero(in_range)
 
         looks_like_ff = n_in_range >= self.min_flatfield_pixel_fraction * image.size
+        
         if looks_like_ff:
-            array_event.trigger.event_type = EventType.FLATFIELD
-            self.log.debug(
-                'Setting event type of event'
-                f' {array_event.index.event_id} to FLATFIELD'
-            )
+            # Tag as FF only events with 2-gains waveforms: both gains are needed for calibration
+            if waveform.ndim == 3:
+                array_event.trigger.event_type = EventType.FLATFIELD
+                self.log.debug(
+                    'Setting event type of event'
+                    f' {array_event.index.event_id} to FLATFIELD'
+                )
+            else:
+                array_event.trigger.event_type = EventType.UNKNOWN
+                self.log.warning(
+                    'Found FF-looking event that has just one gain:'
+                    f'{array_event.index.event_id}. Setting event type to UNKNOWN'
+                )
         elif array_event.trigger.event_type == EventType.FLATFIELD:
             self.log.warning(
-                'Found FF event that does not fulfill FF criteria: %d',
-                array_event.index.event_id,
+                'Found FF event that does not fulfill FF criteria:'
+                f'{array_event.index.event_id}. Setting event type to UNKNOWN' 
             )
             array_event.trigger.event_type = EventType.UNKNOWN