diff --git a/src/cryo_sbi/utils/image_utils.py b/src/cryo_sbi/utils/image_utils.py
index e0a4958..b4b0f12 100644
--- a/src/cryo_sbi/utils/image_utils.py
+++ b/src/cryo_sbi/utils/image_utils.py
@@ -357,7 +357,7 @@ def get_image(self, idx: Union[int, list]):
         """
         assert (
             self._index_map is not None
-        ), "Index map not built. First call build_index_map()"
+        ), "Index map not built. First call build_index_map() or load_index_map()"
         if isinstance(idx, int):
             image = mrc_to_tensor(self.paths[self._path_index[idx]])
             if image.ndim > 2: