Array Index out of bounds Exception

Trace pasted below.

LowLevelException: Error in low-level CAS APIs: array index out of range: 18.
at org.apache.uima.jcas.impl.JCasImpl.checkArrayBounds(JCasImpl.java:1115)
at org.apache.ctakes.typesystem.type.syntax.TopTreebankNode.getTerminals(TopTreebankNode.java:105)
at org.apache.ctakes.constituency.parser.util.TreeUtils.recursivelyCreateStructure(TreeUtils.java:271)
at org.apache.ctakes.constituency.parser.util.TreeUtils.recursivelyCreateStructure(TreeUtils.java:281)
at org.apache.ctakes.constituency.parser.util.TreeUtils.buildAlignedTree(TreeUtils.java:183)
at org.apache.ctakes.constituency.parser.MaxentParserWrapper.createAnnotations(MaxentParserWrapper.java:97)
at org.apache.ctakes.constituency.parser.ae.ConstituencyParser.process(ConstituencyParser.java:62)
at org.apache.uima.analysis_component.JCasAnnotator_ImplBase.process(JCasAnnotator_ImplBase.java:48)
at org.apache.uima.analysis_engine.impl.PrimitiveAnalysisEngine_impl.callAnalysisComponentProcess(PrimitiveAnalysisEngine_impl.java:375)
... 26 more