diff --git a/src/kyupy/techlib.py b/src/kyupy/techlib.py
index 1fb0e39..c2da092 100644
--- a/src/kyupy/techlib.py
+++ b/src/kyupy/techlib.py
@@ -50,13 +50,13 @@ class TechLib:
         assert pin in self.cells[kind][1], f'Unknown pin: {pin} for cell {kind}'
         return self.cells[kind][1][pin][0]
 
-    def pin_name(self, kind, pos, output=False):
+    def pin_name(self, kind: str, pos: int, output=False) -> str:
         """Returns the pin name for a given node kind, list position, and direction."""
         assert kind in self.cells, f'Unknown cell: {kind}'
         for name, (ppos, isout) in self.cells[kind][1].items():
             if isout == output and ppos == pos:
                 return name
-        return None
+        raise ValueError(f'Invalid {"output" if output else "input"} pin position {pos} for cell: {kind}')
 
     def pin_is_output(self, kind, pin):
         """Returns True, if given pin name of a node kind is an output."""