Building jdk13 fails

[Johannes Kingma]

Hello,

I have diablo-jdk13 installed on current. When I try to build jdk13. I 
get this error.

===>  Building for jdk-1.3.1p8_2
# Start of jdk build
i386 Build started:   1.3.1-p8-johannes-031020-21:18
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.

Exiting because of the above error(s).
gmake: *** [sanity] Error 1
*** Error code 2

Stop in /usr/ports/java/jdk13.

Why this error, it should not with diablo-jdk13 installed?