diff --git a/altamisa/isatab/validate_assay_study.py b/altamisa/isatab/validate_assay_study.py
index 9cd6d3b..f68e831 100644
--- a/altamisa/isatab/validate_assay_study.py
+++ b/altamisa/isatab/validate_assay_study.py
@@ -13,7 +13,6 @@
 from . import models
 from ..constants import table_headers, table_restrictions, table_tokens
 from ..exceptions import CriticalIsaValidationWarning, ModerateIsaValidationWarning
-from .helpers import is_ontology_term_ref
 
 __author__ = "Mathias Kuhring <mathias.kuhring@bih-charite.de>"
 
@@ -87,7 +86,7 @@ def validate(self, material: models.Material):
     def _validate_material_annotations(self, material: models.Material):
         # Warn about unnamed materials/data files if there are annotations
         def has_content(value):
-            if is_ontology_term_ref(value):
+            if isinstance(value, models.OntologyTermRef):
                 return value.name or value.accession or value.ontology_name
             else:
                 return value