NPE trying to get StatisticVariable

Hello, I have the following error while running a perf test ? does someone know the root cause ?

15:31:15,958 ERROR [AssertionAddonImpl] Failed adding AssertionItem
java.lang.NullPointerException: Cannot invoke "com.eviware.loadui.api.statistics.StatisticVariable.getStatistic(String, String)" because the return value of "com.eviware.loadui.api.statistics.StatisticHolder.getStatisticVariable(String)" is null
        at com.eviware.loadui.util.serialization.StatisticResolver.getValue(StatisticResolver.java:62) ~[loadui-utils-3.53.0.jar:?]
        at com.eviware.loadui.util.serialization.StatisticResolver.getValue(StatisticResolver.java:1) ~[loadui-utils-3.53.0.jar:?]
        at com.eviware.loadui.impl.assertion.AssertionItemImpl.listenableValueFrom(AssertionItemImpl.java:210) ~[loadui-base-3.53.0.jar:?]
        at com.eviware.loadui.impl.assertion.AssertionItemImpl.<init>(AssertionItemImpl.java:186) ~[loadui-base-3.53.0.jar:?]
        at com.eviware.loadui.impl.assertion.AssertionAddonImpl.initialize(AssertionAddonImpl.java:115) [loadui-base-3.53.0.jar:?]
        at com.eviware.loadui.impl.assertion.AssertionAddonImpl.<init>(AssertionAddonImpl.java:105) [loadui-base-3.53.0.jar:?]