Play 2 scala route controller bytecode does not match all bytecode

I work on Mint 12, Play 2.0 and try to get Proguard to work because my Heroku puncture is too big.

So, I use the xsbt proguard plugin , and it is barfs, because ProGuard does not like the 51 bytecode format. This is ProGuard 4.7, so it should work with 51, but it is not:

[error] {file:/(some path)/*:proguard: java.io.IOException: Can't read
[/(that path)/target/scala-2.9.1/(my app)_2.9.1-1.0-SNAPSHOT.jar]
(Can't process class [controllers/routes$ref.class]
(Unsupported version number [51.0] for class format))

So, I look at that in the bank, and almost all the lines look like this:

./controllers/Application.class:  compiled Java class data, version 49.0 (Java 1.5)

But the router looks like this:

./controllers/routes.class:       compiled Java class data, version 51.0

What would make the router choose a different -target than anything else?

I use JDK7, which comes with Mint, if that matters:

OpenJDK Runtime Environment (IcedTea7 2.0) (7~b147-2.0-0ubuntu0.11.10.1)
OpenJDK 64-Bit Server VM (build 21.0-b17, mixed mode