diff --git a/pytket/phir/routing.py b/pytket/phir/routing.py
index eaabb46..f2d94f1 100644
--- a/pytket/phir/routing.py
+++ b/pytket/phir/routing.py
@@ -9,17 +9,26 @@
 from __future__ import annotations
 
 
+class TransportError(Exception):
+    def __init__(self, a: list[int], b: list[int]):
+        super().__init__(f"Traps different sizes: {len(a)} vs. {len(b)}")
+
+
+class PermutationError(Exception):
+    def __init__(self, lst: list[int]):
+        super().__init__(f"List {lst} is not a permutation of range({len(lst)})")
+
+
 def inverse(lst: list[int]) -> list[int]:
     """Inverse of a permutation list. If a[i] = x, then inverse(a)[x] = i."""
     inv = [-1] * len(lst)
-    for (i, elem) in enumerate(lst):
-        if not 0 <= elem < len(lst):
-            raise ValueError(f"List contains element not in range: {elem}")
-        if inv[elem] != -1:
-            raise ValueError(f"List contains duplicate elements: {lst}")
+    for i, elem in enumerate(lst):
+        if not 0 <= elem < len(lst) or inv[elem] != -1:
+            raise PermutationError(lst)
         inv[elem] = i
     return inv
 
+
 def transport_cost(init: list[int], goal: list[int], swap_cost: float) -> float:
     """Cost of transport from init to goal.
 
@@ -27,8 +36,8 @@ def transport_cost(init: list[int], goal: list[int], swap_cost: float) -> float:
     Transposition Sort, which is the maximum distance that any qubit travels.
     """
     if len(init) != len(goal):
-        raise ValueError(
-            f"init and goal lists have different lengths: {len(init)} vs. {len(goal)}"
-        )
-    n_swaps = max(abs(g - i) for (i, g) in zip(inverse(init), inverse(goal)))
+        raise TransportError(init, goal)
+    n_swaps = max(
+        abs(g - i) for (i, g) in zip(inverse(init), inverse(goal))  # noqa: B905
+    )
     return n_swaps * swap_cost