diff --git a/snirf/pysnirf2.py b/snirf/pysnirf2.py
index 18023b0..811a2e0 100644
--- a/snirf/pysnirf2.py
+++ b/snirf/pysnirf2.py
@@ -576,6 +576,9 @@ def _read_float_array(dataset: h5py.Dataset) -> np.ndarray:
     (27, 1, 'The optional indexed group has no elements'),
     # OK (Severity 0)
     'OK': (28, 0, 'No issues detected'),
+    'CONFLICTING_FIELDS_PRESENT': (
+        29, 3, 'Multiple conflicting fields are present in the same file'
+    )
 }
 
 
@@ -6762,21 +6765,25 @@ def measurementLists_to_measurementList(self):
     def _validate(self, result: ValidationResult):
 
         # Override measurementList/measurementLists validation, only one is required
-        ml = self.measurementList is not None
-        mls = self.measurementLists is not None
+        if self.measurementList:
+            ml = True
+        else:
+            ml = False
+
+        mls = self.measurementLists.sourceIndex is not None
         if (ml and mls):
-            result._add(self.location + '/measurementList', 'OK')
-            result._add(self.location + '/measurementLists', 'OK')
-        elif (ml or mls):
             result._add(self.location + '/measurementList',
-                        ['OPTIONAL_DATASET_MISSING', 'OK'][int(ml)])
+                        'CONFLICTING_FIELDS_PRESENT')
             result._add(self.location + '/measurementLists',
-                        ['OPTIONAL_DATASET_MISSING', 'OK'][int(mls)])
+                        'CONFLICTING_FIELDS_PRESENT')
+        elif (ml or mls):
+            result._add(self.location + '/measurementList', 'OK')
+            result._add(self.location + '/measurementLists', 'OK')
         else:
             result._add(self.location + '/measurementList',
-                        ['REQUIRED_DATASET_MISSING', 'OK'][int(ml)])
+                        'REQUIRED_DATASET_MISSING')
             result._add(self.location + '/measurementLists',
-                        ['REQUIRED_DATASET_MISSING', 'OK'][int(mls)])
+                        'REQUIRED_DATASET_MISSING')
 
         # Check time/dataTimeSeries length agreement
         if all(attr is not None for attr in [self.time, self.dataTimeSeries]):
diff --git a/tests/test.py b/tests/test.py
index 7f3c0e8..b509270 100644
--- a/tests/test.py
+++ b/tests/test.py
@@ -817,6 +817,9 @@ def test_dynamic(self):
 
         assert sizes[1] < sizes[0], 'Dynamically-loaded files not smaller in memory'
 
+    def test_conflicting_fields(self):
+        # TODO
+        pass
 
     def setUp(self):
         if VERBOSE: