ghidra: gradle buildGhidra fails with java version

I’m trying to build ghidra according to the manual. Everything goes fine until I get to

$ gradle buildGhidra

Where I get the error:

Could not target platform: 'Java SE 11' using tool chain: 'JDK 8 (1.8)'

However, my java version seems to be correct:

$ java --version
openjdk 11.0.8 2020-07-14
OpenJDK Runtime Environment (build 11.0.8+10-post-Ubuntu-0ubuntu118.04.1)
OpenJDK 64-Bit Server VM (build 11.0.8+10-post-Ubuntu-0ubuntu118.04.1, mixed mode, sharing)

Can anyone help me to solve this?