diff --git a/src/kyupy/circuit.py b/src/kyupy/circuit.py
index c3e8487..fd8dc05 100644
--- a/src/kyupy/circuit.py
+++ b/src/kyupy/circuit.py
@@ -18,6 +18,8 @@ from typing import Union, Any
 
 import numpy as np
 
+type NestedNumericList = list['int|NestedNumericList']
+type NestedStrIntDict = dict[str,'int|NestedStrIntDict']
 
 class GrowingList[T](list[T]):
     def __setitem__(self, index, value):
@@ -290,10 +292,10 @@ class Circuit:
         return list(self.io_nodes) + [n for n in self.nodes if 'dff' in n.kind.lower()] + [n for n in self.nodes if 'latch' in n.kind.lower()]
 
     def io_locs(self, prefix):
-        """Returns the indices of primary I/Os that start with given name prefix.
+        """Returns a list of indices of primary I/Os that start with given name prefix.
 
         The returned values are used to index into the :py:attr:`io_nodes` array.
-        If only one I/O cell matches the given prefix, a single integer is returned.
+        If only one I/O cell matches the given prefix, a list with a single integer is returned.
         If a bus matches the given prefix, a sorted list of indices is returned.
         Busses are identified by integers in the cell names following the given prefix.
         Lists for bus indices are sorted from LSB (e.g. :code:`data[0]`) to MSB (e.g. :code:`data[31]`).
@@ -311,8 +313,8 @@ class Circuit:
         """
         return self._locs(prefix, self.s_nodes)
 
-    def _locs(self, prefix, nodes:list[Node]) -> Node|list[Any]:  # can return list[list[...]]
-        d_top = dict()
+    def _locs(self, prefix, nodes:list[Node]) -> NestedNumericList:  # can return list[list[...]]
+        d_top: NestedStrIntDict = dict()
         for i, n in enumerate(nodes):
             if m := re.match(fr'({re.escape(prefix)}.*?)((?:[\d_\[\]])*$)', n.name):
                 path = [m[1]] + [int(v) for v in re.split(r'[_\[\]]+', m[2]) if len(v) > 0]
@@ -320,13 +322,14 @@ class Circuit:
                 for j in path[:-1]:
                     d[j] = d.get(j, dict())
                     d = d[j]
+                    assert isinstance(d, dict)
                 d[path[-1]] = i
 
         # sort recursively for multi-dimensional lists.
-        def sorted_values(d): return [sorted_values(v) for k, v in sorted(d.items())] if isinstance(d, dict) else d
+        def sorted_values(d) -> NestedNumericList: return [sorted_values(v) for k, v in sorted(d.items())] if isinstance(d, dict) else d
         l = sorted_values(d_top)
-        while isinstance(l, list) and len(l) == 1: l = l[0]
-        return l #None if isinstance(l, list) and len(l) == 0 else l
+        while isinstance(l, list) and len(l) == 1 and isinstance(l[0], list): l = l[0]
+        return l
 
     @property
     def stats(self):