GelfLayout throws exception if some log event fields are null

2015-07-08 11:07:41,761 ERROR An exception occurred processing Appender Console java.lang.NullPointerException
	at com.fasterxml.jackson.core.io.JsonStringEncoder.quoteAsString(JsonStringEncoder.java:106)
	at org.apache.logging.log4j.core.layout.GelfLayout.toSerializable(GelfLayout.java:217)
	at org.apache.logging.log4j.core.layout.GelfLayout.toByteArray(GelfLayout.java:184)
	at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.append(AbstractOutputStreamAppender.java:108)
	at org.apache.logging.log4j.core.config.AppenderControl.callAppender(AppenderControl.java:99)
	at org.apache.logging.log4j.core.config.LoggerConfig.callAppenders(LoggerConfig.java:430)
	at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:409)
	at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:367)
	at org.apache.logging.log4j.core.Logger.logMessage(Logger.java:112)
	at org.apache.logging.log4j.spi.AbstractLogger.logMessage(AbstractLogger.java:727)
	at org.apache.logging.log4j.spi.AbstractLogger.logIfEnabled(AbstractLogger.java:716)