diff --git a/sktime/registry/_base_classes.py b/sktime/registry/_base_classes.py
index d9b49249bc2..c7819d8bc1b 100644
--- a/sktime/registry/_base_classes.py
+++ b/sktime/registry/_base_classes.py
@@ -399,6 +399,30 @@ def _construct_base_class_register(mixin=False):
     return register
 
 
+def get_base_class_for_str(scitype_str):
+    """Return base class for a given scitype string.
+
+    Parameters
+    ----------
+    scitype_str : str, or list of str
+        scitype shorthand, as in scitype_name field of scitype classes
+
+    Returns
+    -------
+    base_cls : class or list of class
+        base class corresponding to the scitype string,
+        or list of base classes if input was a list
+    """
+    if isinstance(scitype_str, list):
+        return [get_base_class_for_str(s) for s in scitype_str]
+
+    base_classes = _get_base_classes()
+    base_classes += _get_base_classes(mixin=True)
+    base_class_lookup = {cl.get_class_tags()["scitype_name"]: cl for cl in base_classes}
+    base_cls = base_class_lookup[scitype_str].get_base_class()
+    return base_cls
+
+
 def get_base_class_register(mixin=False, include_baseobjs=True):
     """Return register of object scitypes and base classes in sktime.
 
diff --git a/sktime/registry/_lookup.py b/sktime/registry/_lookup.py
index 93894f7e333..b72b7b1c198 100644
--- a/sktime/registry/_lookup.py
+++ b/sktime/registry/_lookup.py
@@ -21,7 +21,10 @@
 import pandas as pd
 from skbase.lookup import all_objects
 
-from sktime.registry._base_classes import get_base_class_lookup, get_obj_scitype_list
+from sktime.registry._base_classes import (
+    get_base_class_for_str,
+    get_obj_scitype_list,
+)
 from sktime.registry._tags import ESTIMATOR_TAG_REGISTER
 
 
@@ -362,7 +365,17 @@ def is_tag_for_type(tag, estimator_types):
 
 
 def _check_estimator_types(estimator_types):
-    """Return list of classes corresponding to type strings."""
+    """Return list of classes corresponding to type strings.
+
+    Parameters
+    ----------
+    estimator_types: str, or list of str
+
+    Returns
+    -------
+    estimator_types: list of classes
+        base classes corresponding to scitype strings in estimator_types
+    """
     estimator_types = deepcopy(estimator_types)
 
     if not isinstance(estimator_types, list):
@@ -370,14 +383,12 @@ def _check_estimator_types(estimator_types):
 
     def _get_err_msg(estimator_type):
         return (
-            f"Parameter `estimator_type` must be None, a string or a list of "
+            f"Parameter `estimator_type` must be a string or a list of "
             f"strings. Valid string values are: "
             f"{get_obj_scitype_list()}, but found: "
             f"{repr(estimator_type)}"
         )
 
-    BASE_CLASS_LOOKUP = get_base_class_lookup()
-
     for i, estimator_type in enumerate(estimator_types):
         if not isinstance(estimator_type, (type, str)):
             raise ValueError(
@@ -386,8 +397,7 @@ def _get_err_msg(estimator_type):
         if isinstance(estimator_type, str):
             if estimator_type not in get_obj_scitype_list():
                 raise ValueError(_get_err_msg(estimator_type))
-            estimator_type = BASE_CLASS_LOOKUP[estimator_type]
-            estimator_types[i] = estimator_type
+            estimator_types[i] = get_base_class_for_str(estimator_type)
         elif isinstance(estimator_type, type):
             pass
         else: