"return.falseproblem;" is not a "false" error.

[moufmouf]

The function can legitimately return false!

From the documentation:

<!ENTITY return.falseproblem '<warning xmlns="http://docbook.org/ns/docbook"><simpara>This function may
return Boolean &false;, but may also return a non-Boolean value which
evaluates to &false;. Please read the section on <link
linkend="language.types.boolean">Booleans</link> for more
information. Use <link linkend="language.operators.comparison">the ===
operator</link> for testing the return value of this
function.</simpara></warning>'>