diff --git a/ciphers/a1z26.py b/ciphers/a1z26.py
index a1377ea6d397..102cef4e424e 100644
--- a/ciphers/a1z26.py
+++ b/ciphers/a1z26.py
@@ -13,7 +13,13 @@ def encode(plain: str) -> list[int]:
     """
     >>> encode("myname")
     [13, 25, 14, 1, 13, 5]
+    >>> encode("ABCD")
+    Traceback (most recent call last):
+        ...
+    ValueError: plain must contain only lowercase letters (a-z)
     """
+    if not plain.islower() or not plain.isalpha():
+        raise ValueError("plain must contain only lowercase letters (a-z)")
     return [ord(elem) - 96 for elem in plain]