HTTP ERROR 500

Problem accessing /test_C/param. Reason: 

    Unexpected error: Improper use? Potential bug? Your feedback is welcome:
Contact: basex-talk@mailman.uni-konstanz.de
Version: BaseX 7.7
Java: Sun Microsystems Inc., 1.6.0_27
OS: Linux, amd64
Stack Trace: 
java.lang.NullPointerException
	at org.basex.query.expr.Arr.has(Arr.java:46)
	at org.basex.query.func.StaticFuncCall.has(StaticFuncCall.java:142)
	at org.basex.query.QueryContext.mainModule(QueryContext.java:203)
	at org.basex.http.restxq.RestXqResponse.create(RestXqResponse.java:68)
	at org.basex.http.restxq.RestXqModule.process(RestXqModule.java:108)
	at org.basex.http.restxq.RestXqFunction.process(RestXqFunction.java:100)
	at org.basex.http.restxq.RestXqServlet.run(RestXqServlet.java:31)
	at org.basex.http.BaseXServlet.service(BaseXServlet.java:58)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
	at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:684)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:457)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137)
	at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:557)
	at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231)
	at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1075)
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:384)
	at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:193)
	at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1009)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
	at org.eclipse.jetty.server.Server.handle(Server.java:370)
	at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:489)
	at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:949)
	at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:1011)
	at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:644)
	at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
	at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
	at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:668)
	at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:52)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
	at java.lang.Thread.run(Thread.java:679)

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