diff --git a/src/aind_data_schema/core/procedures.py b/src/aind_data_schema/core/procedures.py
index 388bbf8ef..a31d487e3 100644
--- a/src/aind_data_schema/core/procedures.py
+++ b/src/aind_data_schema/core/procedures.py
@@ -97,15 +97,15 @@ def validate_subject_specimen_ids(self):
         # Return if no specimen procedures
         if self.specimen_procedures:
             subject_id = self.subject_id
-            specimen_id_vars = [spec_proc.specimen_id for spec_proc in self.specimen_procedures]
-            specimen_ids = []
-            for spec_id_var in specimen_id_vars:
-                if isinstance(spec_id_var, str):
-                    specimen_ids.append(spec_id_var)
+            flat_specimen_ids = []
+            for spec_proc in self.specimen_procedures:
+                sid = spec_proc.specimen_id
+                if isinstance(sid, list):
+                    flat_specimen_ids.extend(sid)
                 else:
-                    specimen_ids.extend(spec_id_var)
+                    flat_specimen_ids.append(sid)
 
-            if any(not subject_specimen_id_compatibility(subject_id, spec_id) for spec_id in specimen_ids):
+            if any(not subject_specimen_id_compatibility(subject_id, spec_id) for spec_id in flat_specimen_ids):
                 raise ValueError("specimen_id must be an extension of the subject_id.")
 
         return self
diff --git a/tests/test_procedures.py b/tests/test_procedures.py
index 4554c4c8a..3121a0d06 100644
--- a/tests/test_procedures.py
+++ b/tests/test_procedures.py
@@ -28,6 +28,7 @@
     PlanarSection,
     PlanarSectioning,
     Section,
+    Sectioning,
     SectionOrientation,
     SpecimenProcedure,
 )
@@ -332,6 +333,19 @@ def test_validate_procedure_type(self):
             )
         )
 
+        self.assertIsNotNone(
+            SpecimenProcedure(
+                specimen_id="1000",
+                procedure_type="Sectioning",
+                start_date=date.fromisoformat("2020-10-10"),
+                end_date=date.fromisoformat("2020-10-11"),
+                experimenters=["Mam Moth"],
+                protocol_id=["10"],
+                notes=None,
+                procedure_details=[Sectioning(sections=[Section(output_specimen_id="1000_spinal")])],
+            )
+        )
+
     def test_validate_procedure_type_multiple(self):
         """Test that error thrown when multiple types are passed to procedure_details"""