diff --git a/esmvalcore/_recipe/from_datasets.py b/esmvalcore/_recipe/from_datasets.py
index 8bd33fd5e9..3fbbce8908 100644
--- a/esmvalcore/_recipe/from_datasets.py
+++ b/esmvalcore/_recipe/from_datasets.py
@@ -4,9 +4,10 @@
 import itertools
 import logging
 import re
+from collections.abc import Iterable, Mapping, Sequence
 from functools import partial
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Dict, Iterable, Mapping, Sequence
+from typing import TYPE_CHECKING, Any, Dict
 
 from nested_lookup import nested_delete
 
@@ -99,10 +100,12 @@ def _move_datasets_up(recipe: Recipe) -> Recipe:
 
 def _to_frozen(item):
     """Return a frozen and sorted copy of nested dicts and lists."""
-    if isinstance(item, list):
-        return tuple(sorted(_to_frozen(elem) for elem in item))
-    if isinstance(item, dict):
+    if isinstance(item, str):
+        return item
+    if isinstance(item, Mapping):
         return tuple(sorted((k, _to_frozen(v)) for k, v in item.items()))
+    if isinstance(item, Iterable):
+        return tuple(sorted(_to_frozen(elem) for elem in item))
     return item