if you omit gradle version in wrapper task, like this:

task wrapper(type: Wrapper) {
}

all you get is:

:wrapper

FAILURE: Build failed with an exception.

• Where:
  Build file '/home/tomek/workspace/gradle_0.9/build.gradle'

• What went wrong:
  Execution failed for task ':wrapper'.
  Cause: java.lang.NullPointerException (no error message)

• Try:
  Run with -s or -d option to get more details. Run with -S option to get the full (very verbose) stacktrace.

BUILD FAILED

would be much better to get a meaningful error message, something like: "gradleVersion not specified"