diff --git a/gufe/mapping/ligandatommapping.py b/gufe/mapping/ligandatommapping.py
index 75c81969..e7acb6df 100644
--- a/gufe/mapping/ligandatommapping.py
+++ b/gufe/mapping/ligandatommapping.py
@@ -55,10 +55,10 @@ def __init__(
         for i, j in componentA_to_componentB.items():
             if not (0 <= i < nA):
                 raise ValueError(f"Got invalid index for ComponentA ({i}); "
-                                 f"must be 0 <= n <= {nA}")
+                                 f"must be 0 <= n < {nA}")
             if not (0 <= j < nB):
                 raise ValueError(f"Got invalid index for ComponentB ({i}); "
-                                 f"must be 0 <= n <= {nB}")
+                                 f"must be 0 <= n < {nB}")
 
         self._compA_to_compB = componentA_to_componentB