This issue is ultra trivial to fix.
When a non-null @In attribute is required, but not found, the following message is shown:
"In attribute requires non-null value"
However, when a non-null @Out attribute is required, but not found, the message is shown as:
"@Out attribute requires non-null value"
The @ sign is very important as it lets the developer know we are talking about an annotation. Otherwise, the word "In" is ambiguous.