diff --git a/fsl/data/atlases.py b/fsl/data/atlases.py
index 3bd1da02bc6ecea647dae1f14f5130f3def17183..51d2f507ef8898b279c14c3b790187b9eabbe1e9 100644
--- a/fsl/data/atlases.py
+++ b/fsl/data/atlases.py
@@ -570,7 +570,9 @@ class AtlasDescription(object):
         """Find an :class:`.AtlasLabel` either by ``index``, or by ``value``.
 
         Exactly one of ``index`` or ``value`` may be specified - a
-        ``ValueError`` is raised otherwise.
+        ``ValueError`` is raised otherwise. If an invalid ``index`` or
+        ``value`` is specified, an ``IndexError`` or ``KeyError`` will be
+        raised.
         """
         if (index is     None and value is     None) or \
            (index is not None and value is not None):