diff --git a/apps/datasetmanager/dataset_data.py b/apps/datasetmanager/dataset_data.py
index 0c7603c..680b9e4 100644
--- a/apps/datasetmanager/dataset_data.py
+++ b/apps/datasetmanager/dataset_data.py
@@ -47,7 +47,7 @@ def from_json(data):
         obj = json.loads(data, object_pairs_hook=OrderedDict)
         return DatasetData(data=obj)
 
-    def validate(self, time_start, time_end):
+    def validate(self, time_start, time_end, class_id):
         if not isinstance(self.data, dict):
             raise ValidationError("Data field needs to be a dictionary.")
 
@@ -69,6 +69,10 @@ def validate(self, time_start, time_end):
                 raise ValidationError("Element %d of data.table has not 'row' field." % index)
             if 'individual' not in value:
                 raise ValidationError("Element %d of data.table has not 'individual' field." % index)
+            if class_id != 7:
+                if not isinstance(value['individual'], int):
+                    raise ValidationError("Individual of Element %d of data.table is not an integer. "
+                                          "Please use class 'custom' to provide strings." % index)
             if 'values' not in value:
                 raise ValidationError("Element %d of data.table has not 'values' field." % index)
             if not isinstance(value['values'], dict):
diff --git a/apps/datasetmanager/fields.py b/apps/datasetmanager/fields.py
index 6090ac2..d4dcebf 100644
--- a/apps/datasetmanager/fields.py
+++ b/apps/datasetmanager/fields.py
@@ -18,12 +18,12 @@ def field_from_native(self, data, files, field_name, into):
         if 'data' not in data:
             raise ValidationError("Field 'data' is required.")
 
-        dataset = DatasetData(data=data[field_name])
-        dataset.validate(into['time_start'], into['time_end'])
-
         if 'class_id' not in data:
             raise ValidationError('Field class_id is missing.')
 
+        dataset = DatasetData(data=data[field_name])
+        dataset.validate(into['time_start'], into['time_end'], data['class_id'])
+
         if data['class_id'] == 7:
             dataset.create_individuals()