Plugin fails compatibility testing using gradleTest v 3.0.0 and Java 8

Using gradleTest 3.0.0 and Java 8 generates the following error:

Caused by: org.gradle.api.GradleException: Could not generate a proxy class for class org.ysb33r.gradle.terraform.internal.DefaultTerraformSourceSets.
	at org.gradle.api.internal.AbstractClassGenerator.generateUnderLock(AbstractClassGenerator.java:228)
	at org.gradle.api.internal.AbstractClassGenerator.generate(AbstractClassGenerator.java:80)
	at org.gradle.api.internal.ClassGeneratorBackedInstantiator.newInstance(ClassGeneratorBackedInstantiator.java:36)
	at org.gradle.api.internal.plugins.DefaultConvention.instantiate(DefaultConvention.java:264)
	at org.gradle.api.internal.plugins.DefaultConvention.create(DefaultConvention.java:156)
	at org.gradle.api.internal.plugins.DefaultConvention.create(DefaultConvention.java:151)
	at org.ysb33r.gradle.terraform.plugins.TerraformBasePlugin.createTerraformSourceSetsExtension(TerraformBasePlugin.groovy:88)
	at org.ysb33r.gradle.terraform.plugins.TerraformBasePlugin.apply(TerraformBasePlugin.groovy:66)
	at org.ysb33r.gradle.terraform.plugins.TerraformBasePlugin.apply(TerraformBasePlugin.groovy)
	at org.gradle.api.internal.plugins.ImperativeOnlyPluginTarget.applyImperative(ImperativeOnlyPluginTarget.java:42)
	at org.gradle.api.internal.plugins.RuleBasedPluginTarget.applyImperative(RuleBasedPluginTarget.java:50)
	at org.gradle.api.internal.plugins.DefaultPluginManager.addPlugin(DefaultPluginManager.java:163)
	at org.gradle.api.internal.plugins.DefaultPluginManager.access$200(DefaultPluginManager.java:46)
	at org.gradle.api.internal.plugins.DefaultPluginManager$AddPluginBuildOperation.run(DefaultPluginManager.java:251)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:300)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:292)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:174)
	at org.gradle.internal.operations.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:90)
	at org.gradle.internal.operations.DelegatingBuildOperationExecutor.run(DelegatingBuildOperationExecutor.java:31)
	at org.gradle.api.internal.plugins.DefaultPluginManager.doApply(DefaultPluginManager.java:143)

This issue started with v 3.0.0-alpha.1 of gradleTest.