diff --git a/src/anemoi/datasets/data/dataset.py b/src/anemoi/datasets/data/dataset.py
index aa09d4ca..185cf297 100644
--- a/src/anemoi/datasets/data/dataset.py
+++ b/src/anemoi/datasets/data/dataset.py
@@ -245,7 +245,8 @@ def _drop_to_columns(self, vars):
         if not isinstance(vars, (list, tuple, set)):
             vars = [vars]
 
-        assert set(vars) <= set(self.name_to_index)
+        if not set(vars) <= set(self.name_to_index):
+            raise ValueError(f"drop: unknown variables: {set(vars) - set(self.name_to_index)}")
 
         return sorted([v for k, v in self.name_to_index.items() if k not in vars])
 
diff --git a/src/anemoi/datasets/data/join.py b/src/anemoi/datasets/data/join.py
index 3f97c38a..6b7de3e6 100644
--- a/src/anemoi/datasets/data/join.py
+++ b/src/anemoi/datasets/data/join.py
@@ -124,7 +124,14 @@ def variables_metadata(self):
                 if v in md:
                     result[v] = md[v]
 
-        assert len(result) == len(variables), (result, variables)
+        if len(result) != len(variables):
+            LOG.error("Some variables are missing metadata.")
+            for v in variables:
+                if v not in result:
+                    LOG.error("Missing metadata for %r.", v)
+
+            raise ValueError("Some variables are missing metadata.")
+
         return result
 
     @cached_property