The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
The description contains an invalid control statement.
A control statement is a line starting with a dot (.). The only control statement is defined by the policy is a single dot denoting an empty line.
The "empty-line" control statement does not permit any characters following it on the same line. Therefore, the line must consist entirely of a space followed by a dot.
Please refer to Debian Policy Manual section 5.6.13 for details.