Variable substitution doens't work with the Bamboo labeler plugin

java.lang.NullPointerException
	at java.util.HashMap.<init>(HashMap.java:223)
	at com.google.common.collect.Maps.newHashMap(Maps.java:136)
	at com.atlassian.bamboo.variable.CustomVariableContextImpl.filterVariableContextByName(CustomVariableContextImpl.java:269)
	at com.atlassian.bamboo.variable.CustomVariableContextImpl.substituteBambooVariables(CustomVariableContextImpl.java:159)
	at com.atlassian.bamboo.variable.CustomVariableContextImpl.substituteString(CustomVariableContextImpl.java:129)
	at com.atlassian.bamboo.build.VariableSubstitutionBeanImpl.substituteBambooVariables(VariableSubstitutionBeanImpl.java:51)
	at com.atlassian.bamboo.plugins.labeller.BuildLabeller.replaceVariables(BuildLabeller.java:310)
	at com.atlassian.bamboo.plugins.labeller.BuildLabeller.labelBuildResult(BuildLabeller.java:284)
	at com.atlassian.bamboo.plugins.labeller.BuildLabeller.call(BuildLabeller.java:85)
	at sun.reflect.GeneratedMethodAccessor436.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)