Error "Could not find method implementation()" when building src project

[vaci]

I'm attempting to use gradle2nix to build a large project, https://github.com/real-logic/aeron.

Most of the dependencies seem to be processed successfully, but I get an odd error with the buildSrc sub-project, which has the following build.gradle file:

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.asciidoctor:asciidoctorj:2.5.2'
    implementation 'org.eclipse.jgit:org.eclipse.jgit:5.13.0.202109080827-r'
}

The error is:
Caused by: org.gradle.internal.metaobject.AbstractDynamicObject$CustomMessageMissingMethodException: Could not find method implementation() for arguments [org.asciidoctor:asciidoctorj:2.5.2] on object of type org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.

Reversing the order of the dependencies results in the error being triggered by the jgit project instead.

This seems to be a pretty normal dependency clause? I'm not particularly familiar with gradle, so there may be an obvious cause that I'm missing.