diff --git a/utils/datasets/datasets.py b/utils/datasets/datasets.py
index 8ad8c68..2971b93 100644
--- a/utils/datasets/datasets.py
+++ b/utils/datasets/datasets.py
@@ -55,7 +55,8 @@ def name(self):
     @property
     def n_categories(self):
         if self._n_categories is None:
-            assert self.training_set.target.dtype in (int, str), \
+            target_dtype = self.training_set.target.dtype
+            assert (np.issubdtype(target_dtype, int) or np.issubdtype(target_dtype, str)), \
                 'n_categories is only a valid attribute when target data is int or str.  It is %s' \
                 % (self.training_set.target.dtype, )
             self._n_categories = len(np.unique(self.training_set.target))