Combine javadocs of subprojects: too many module declarations

I use this to try to create an aggregate javavadoc of all subprojects, alongside the jar that combines all subproject jars (next up would be combined sources):

task javadocs(type: Javadoc) {
	source subprojects.collect { project ->
	println buildDir
	destinationDir = new File(buildDir, 'javadoc')

This task fails with the error too many module declarations found. Each of my subprojects has a separate that now get listed. What would be a workaround here?