Download dependencies when using Maven BOM file

I’m using Gradle 5.2.1 and have a Gradle build file containing Maven’s BOM definition (using platform function as explained here).

When I try to download the dependencies locally using a dedicated copy task it fails with the following exception:

org.gradle.api.tasks.TaskExecutionException: Execution failed for task ':downloadDeps'.
        at org.gradle.api.internal.tasks.execution.CatchExceptionTaskExecuter.execute(CatchExceptionTaskExecuter.java:38)
        at org.gradle.api.internal.tasks.execution.EventFiringTaskExecuter$1.executeTask(EventFiringTaskExecuter.java:63)
        at org.gradle.api.internal.tasks.execution.EventFiringTaskExecuter$1.call(EventFiringTaskExecuter.java:49)
        at org.gradle.api.internal.tasks.execution.EventFiringTaskExecuter$1.call(EventFiringTaskExecuter.java:46)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$CallableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:416)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$CallableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:406)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$1.execute(DefaultBuildOperationExecutor.java:165)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:250)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:158)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.call(DefaultBuildOperationExecutor.java:102)
        at org.gradle.internal.operations.DelegatingBuildOperationExecutor.call(DelegatingBuildOperationExecutor.java:36)
        at org.gradle.api.internal.tasks.execution.EventFiringTaskExecuter.execute(EventFiringTaskExecuter.java:46)
        at org.gradle.execution.plan.LocalTaskNodeExecutor.execute(LocalTaskNodeExecutor.java:43)
        at org.gradle.execution.taskgraph.DefaultTaskExecutionGraph$InvokeNodeExecutorsAction.execute(DefaultTaskExecutionGraph.java:355)
        at org.gradle.execution.taskgraph.DefaultTaskExecutionGraph$InvokeNodeExecutorsAction.execute(DefaultTaskExecutionGraph.java:343)
        at org.gradle.execution.taskgraph.DefaultTaskExecutionGraph$BuildOperationAwareExecutionAction.execute(DefaultTaskExecutionGraph.java:336)
        at org.gradle.execution.taskgraph.DefaultTaskExecutionGraph$BuildOperationAwareExecutionAction.execute(DefaultTaskExecutionGraph.java:322)
        at org.gradle.execution.plan.DefaultPlanExecutor$ExecutorWorker$1.execute(DefaultPlanExecutor.java:134)
        at org.gradle.execution.plan.DefaultPlanExecutor$ExecutorWorker$1.execute(DefaultPlanExecutor.java:129)
        at org.gradle.execution.plan.DefaultPlanExecutor$ExecutorWorker.execute(DefaultPlanExecutor.java:202)
        at org.gradle.execution.plan.DefaultPlanExecutor$ExecutorWorker.executeNextNode(DefaultPlanExecutor.java:193)
        at org.gradle.execution.plan.DefaultPlanExecutor$ExecutorWorker.run(DefaultPlanExecutor.java:129)
        at org.gradle.internal.concurrent.ExecutorPolicy$CatchAndRecordFailures.onExecute(ExecutorPolicy.java:63)
        at org.gradle.internal.concurrent.ManagedExecutorImpl$1.run(ManagedExecutorImpl.java:46)
        at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:55)
Caused by: org.gradle.api.internal.artifacts.ivyservice.DefaultLenientConfiguration$ArtifactResolveException: Could not resolve all files for configuration ':compile'.
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.rethrowFailure(DefaultConfiguration.java:1176)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.access$2100(DefaultConfiguration.java:136)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration$ConfigurationFileCollection.getFiles(DefaultConfiguration.java:1151)
        at org.gradle.api.internal.file.AbstractFileCollection.iterator(AbstractFileCollection.java:72)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.iterator(DefaultConfiguration.java:463)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext$FileTreeConverter.convertInto(DefaultFileCollectionResolveContext.java:209)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext.doResolve(DefaultFileCollectionResolveContext.java:114)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext.resolveAsFileTrees(DefaultFileCollectionResolveContext.java:85)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext$FileTreeConverter.convertInto(DefaultFileCollectionResolveContext.java:195)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext.doResolve(DefaultFileCollectionResolveContext.java:109)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext.resolveAsFileTrees(DefaultFileCollectionResolveContext.java:85)
        at org.gradle.api.internal.file.CompositeFileCollection$1.visitContents(CompositeFileCollection.java:130)
        at org.gradle.api.internal.file.CompositeFileTree$FilteredFileTree.visitContents(CompositeFileTree.java:120)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext.resolveNested(DefaultFileCollectionResolveContext.java:144)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext.doResolve(DefaultFileCollectionResolveContext.java:112)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext.resolveAsFileCollections(DefaultFileCollectionResolveContext.java:92)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext$FileCollectionConverter.convertInto(DefaultFileCollectionResolveContext.java:164)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext.doResolve(DefaultFileCollectionResolveContext.java:109)
        at org.gradle.api.internal.file.collections.DefaultFileCollectionResolveContext.resolveAsFileCollections(DefaultFileCollectionResolveContext.java:92)
        at org.gradle.api.internal.file.CompositeFileCollection.getSourceCollections(CompositeFileCollection.java:192)
        at org.gradle.api.internal.file.CompositeFileCollection.isEmpty(CompositeFileCollection.java:98)
        at org.gradle.api.internal.tasks.execution.SkipEmptySourceFilesTaskExecuter.execute(SkipEmptySourceFilesTaskExecuter.java:63)
        at org.gradle.api.internal.tasks.execution.ResolveBeforeExecutionOutputsTaskExecuter.execute(ResolveBeforeExecutionOutputsTaskExecuter.java:67)
        at org.gradle.api.internal.tasks.execution.ResolveAfterPreviousExecutionStateTaskExecuter.execute(ResolveAfterPreviousExecutionStateTaskExecuter.java:46)
        at org.gradle.api.internal.tasks.execution.CleanupStaleOutputsExecuter.execute(CleanupStaleOutputsExecuter.java:93)
        at org.gradle.api.internal.tasks.execution.FinalizePropertiesTaskExecuter.execute(FinalizePropertiesTaskExecuter.java:45)
        at org.gradle.api.internal.tasks.execution.ResolveTaskExecutionModeExecuter.execute(ResolveTaskExecutionModeExecuter.java:94)
        at org.gradle.api.internal.tasks.execution.SkipTaskWithNoActionsExecuter.execute(SkipTaskWithNoActionsExecuter.java:57)
        at org.gradle.api.internal.tasks.execution.SkipOnlyIfTaskExecuter.execute(SkipOnlyIfTaskExecuter.java:56)
        at org.gradle.api.internal.tasks.execution.CatchExceptionTaskExecuter.execute(CatchExceptionTaskExecuter.java:36)
        ... 24 more
Caused by: org.gradle.internal.resolve.ModuleVersionNotFoundException: Could not find org.springframework.boot:spring-boot-starter:.
Required by:
    project :
        at org.gradle.internal.resolve.result.DefaultBuildableComponentResolveResult.notFound(DefaultBuildableComponentResolveResult.java:42)
        at org.gradle.api.internal.artifacts.ivyservice.ivyresolve.RepositoryChainComponentMetaDataResolver.resolveModule(RepositoryChainComponentMetaDataResolver.java:108)
        at org.gradle.api.internal.artifacts.ivyservice.ivyresolve.RepositoryChainComponentMetaDataResolver.resolve(RepositoryChainComponentMetaDataResolver.java:63)
        at org.gradle.api.internal.artifacts.ivyservice.resolveengine.ComponentResolversChain$ComponentMetaDataResolverChain.resolve(ComponentResolversChain.java:95)
        at org.gradle.api.internal.artifacts.ivyservice.clientmodule.ClientModuleResolver.resolve(ClientModuleResolver.java:63)
        at org.gradle.api.internal.artifacts.ivyservice.resolveengine.graph.builder.ComponentState.resolve(ComponentState.java:189)
        at org.gradle.api.internal.artifacts.ivyservice.resolveengine.graph.builder.ComponentState.getMetadata(ComponentState.java:139)
        at org.gradle.api.internal.artifacts.ivyservice.resolveengine.graph.builder.EdgeState.calculateTargetConfigurations(EdgeState.java:175)
        at org.gradle.api.internal.artifacts.ivyservice.resolveengine.graph.builder.EdgeState.attachToTargetConfigurations(EdgeState.java:131)
        at org.gradle.api.internal.artifacts.ivyservice.resolveengine.graph.builder.DependencyGraphBuilder.attachToTargetRevisionsSerially(DependencyGraphBuilder.java:331)
        at org.gradle.api.internal.artifacts.ivyservice.resolveengine.graph.builder.DependencyGraphBuilder.resolveEdges(DependencyGraphBuilder.java:230)
        at org.gradle.api.internal.artifacts.ivyservice.resolveengine.graph.builder.DependencyGraphBuilder.traverseGraph(DependencyGraphBuilder.java:170)
        at org.gradle.api.internal.artifacts.ivyservice.resolveengine.graph.builder.DependencyGraphBuilder.resolve(DependencyGraphBuilder.java:131)
        at org.gradle.api.internal.artifacts.ivyservice.resolveengine.DefaultArtifactDependencyResolver.resolve(DefaultArtifactDependencyResolver.java:121)
        at org.gradle.api.internal.artifacts.ivyservice.DefaultConfigurationResolver.resolveGraph(DefaultConfigurationResolver.java:171)
        at org.gradle.api.internal.artifacts.ivyservice.ShortCircuitEmptyConfigurationResolver.resolveGraph(ShortCircuitEmptyConfigurationResolver.java:86)
        at org.gradle.api.internal.artifacts.ivyservice.ErrorHandlingConfigurationResolver.resolveGraph(ErrorHandlingConfigurationResolver.java:73)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration$7.run(DefaultConfiguration.java:581)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:402)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:394)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor$1.execute(DefaultBuildOperationExecutor.java:165)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:250)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:158)
        at org.gradle.internal.operations.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:92)
        at org.gradle.internal.operations.DelegatingBuildOperationExecutor.run(DelegatingBuildOperationExecutor.java:31)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.resolveGraphIfRequired(DefaultConfiguration.java:572)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.access$600(DefaultConfiguration.java:136)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration$6.run(DefaultConfiguration.java:552)
        at org.gradle.api.internal.project.DefaultProjectStateRegistry$SafeExclusiveLockImpl.withLock(DefaultProjectStateRegistry.java:244)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.resolveExclusively(DefaultConfiguration.java:548)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.resolveToStateOrLater(DefaultConfiguration.java:543)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.access$2200(DefaultConfiguration.java:136)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration$ConfigurationFileCollection.getSelectedArtifacts(DefaultConfiguration.java:1159)
        at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration$ConfigurationFileCollection.getFiles(DefaultConfiguration.java:1148)
        ... 51 more

It works as expected in case I replace the dependency and add the dependency version (commented out line in the sample build file below).

Can you please help me understand how can I get it to work as expected and download the dependencies locally?

Sample Gradle build file:

plugins {
    id 'java'
}

repositories {
    maven {
        url = 'http://repo.maven.apache.org/maven2'
    }
}

ext {
    springBootVersion = '2.1.2.RELEASE'
}

dependencies {
    implementation platform("org.springframework.boot:spring-boot-dependencies:${springBootVersion}")

//    compile "org.springframework.boot:spring-boot-starter:${springBootVersion}"
    compile "org.springframework.boot:spring-boot-starter"
}

task downloadDeps(type: Copy) {
        from configurations.compile
        into "${project.rootDir}/deps"
}