For both, javadoc states that null values are ignored and one should pass an empty String.

Actual behaviors are different:

• FluentStringsMap accepts null values, as of Error reconstructing URI request when a parameter value is empty #234. FluentStringsMap is used for query params and form params.
• FluentCaseInsensitiveStringsMap still turns null into empty Strings, why not... FluentCaseInsensitiveStringsMap is only used for headers.