SslHttpServerContextFactory does not support private key only PEM content

[tazmaniax]

When trying use the private key generated by LetsEncrypt the following exception occurs java.lang.ClassCastException: class org.bouncycastle.asn1.pkcs.PrivateKeyInfo cannot be cast to class org.bouncycastle.openssl.PEMKeyPair

~        _            _ 
~  _ __ | | __ _ _  _| |
~ | '_ \| |/ _' | || |_|
~ |  __/|_|\____|\__ (_)
~ |_|            |__/   
~
~ play! 1.5.x-1.5.2-19-ga67b35e9, https://www.playframework.com
~ framework ID is devdb
~
~ Ctrl+C to stop
~ 
~ using java version "11.0.2"
Listening for transport dt_socket at address: 8000
00:52:14,175 [main] WARN  ~ You're running Play! in DEV mode
00:52:14,284 [main] INFO  ~ Listening for HTTP on port 9000 (Waiting a first request to start) ...
00:52:14,291 [main] INFO  ~ Listening for HTTPS on port 9443 (Waiting a first request to start) ...
~ Server is up and running
00:54:08,547 [New I/O server boss #34] ERROR ~ Failed to initialize PEMKeyManager from file conf/privkey.pem
java.lang.ClassCastException: class org.bouncycastle.asn1.pkcs.PrivateKeyInfo cannot be cast to class org.bouncycastle.openssl.PEMKeyPair (org.bouncycastle.asn1.pkcs.PrivateKeyInfo and org.bouncycastle.openssl.PEMKeyPair are in unnamed module of loader 'app')
	at play.server.ssl.SslHttpServerContextFactory$PEMKeyManager.<init>(SslHttpServerContextFactory.java:101)
	at play.server.ssl.SslHttpServerContextFactory$PEMKeyManager.<clinit>(SslHttpServerContextFactory.java:84)
	at play.server.ssl.SslHttpServerContextFactory.<clinit>(SslHttpServerContextFactory.java:52)
	at play.server.ssl.SslHttpServerPipelineFactory.getPipeline(SslHttpServerPipelineFactory.java:30)
	at org.jboss.netty.channel.socket.nio.NioServerBoss.registerAcceptedChannel(NioServerBoss.java:134)
	at org.jboss.netty.channel.socket.nio.NioServerBoss.process(NioServerBoss.java:104)
	at org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:337)
	at org.jboss.netty.channel.socket.nio.NioServerBoss.run(NioServerBoss.java:42)
	at org.jboss.netty.util.ThreadRenamingRunnable.run(ThreadRenamingRunnable.java:108)
	at org.jboss.netty.util.internal.DeadLockProofWorker$1.run(DeadLockProofWorker.java:42)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)

Play Version (1.5.x / etc)

1.5.x-1.5.2-19-ga67b35e9

Operating System (Ubuntu 15.10 / MacOS 10.10 / Windows 10)

macOS 10.14.4 Beta (18E205e)

JDK (Oracle 1.8.0_72, OpenJDK 1.8.x, Azul Zing)

11.0.2

Reproducible Test Case

Would have but didn't have another LetsEncrypt private key apart from my own that I can't give out.