ERROR: BOOTDIR environment variable

[Rommel B. Ikeda]

I am trying to install Java from my /usr/ports/java/jdk13 ports.
The error message below came out and I do not know to do with it.

ERROR: Your BOOTDIR environment variable does not point
        to a valid Java 2 SDK for bootstrapping this build.
        A Java 2 SDK 1.3.1 build must be bootstrapped against any
        1.3 build. Please update your ALT_BOOTDIR setting, or
        just unset it, and start your build again.

Can anyone give mne hints and advice on this?
Thank you in advance.

Rommel Ikeda