diff --git a/peps/pep-0639.rst b/peps/pep-0639.rst
index 8556c016e..46354348c 100644
--- a/peps/pep-0639.rst
+++ b/peps/pep-0639.rst
@@ -540,7 +540,7 @@ as specified below:
 
 Any characters or character sequences not covered by this specification are
 invalid. Projects MUST NOT use such values.
-Tools consuming this field MAY reject invalid values with an error.
+Tools consuming this field SHOULD reject invalid values with an error.
 
 Tools MUST assume that license file content is valid UTF-8 encoded text,
 and SHOULD validate this and raise an error if it is not.