diff --git a/src/argus/incident/validators.py b/src/argus/incident/validators.py
index ca59db01f..ec97207ea 100644
--- a/src/argus/incident/validators.py
+++ b/src/argus/incident/validators.py
@@ -29,7 +29,8 @@ def validate_key(value):
 key_value_re = _lazy_re_compile(r"^[a-z0-9_]+=.+?\Z")
 key_value_validator = RegexValidator(
     key_value_re,
-    message='Enter a valid key-value construct: "key=value", where key and value are strings.',
+    message=f'Enter a valid key-value construct: "key=value", where key and value are strings.'
+            f'The key must consist of lowercase letters, numbers and underscores.',
     code="invalid",
 )