Janusgraph can't get vertex label for simple traversals

[baritra]

java.lang.IllegalArgumentException",throwable.msg="expected one element but was: <Artifact, Artifact>",throwable.stack="
	at com.google.common.collect.Iterators.getOnlyElement(Iterators.java:315)
	at com.google.common.collect.Iterators.getOnlyElement(Iterators.java:327)
	at com.google.common.collect.Iterables.getOnlyElement(Iterables.java:268)
	at org.janusgraph.graphdb.vertices.AbstractVertex.getVertexLabelInternal(AbstractVertex.java:125)
	at org.janusgraph.graphdb.vertices.AbstractVertex.vertexLabel(AbstractVertex.java:130)
	at org.janusgraph.graphdb.vertices.AbstractVertex.label(AbstractVertex.java:121)
	at org.janusgraph.graphdb.types.system.ImplicitKey.computeProperty(ImplicitKey.java:83)
	at org.janusgraph.graphdb.query.vertex.BasicVertexCentricQueryBuilder.executeImplicitKeyQuery(BasicVertexCentricQueryBuilder.java:211)
	at org.janusgraph.graphdb.query.vertex.VertexCentricQueryBuilder.properties(VertexCentricQueryBuilder.java:99)
	at org.janusgraph.graphdb.util.ElementHelper.getValues(ElementHelper.java:41)
	at org.janusgraph.graphdb.query.condition.PredicateCondition.evaluate(PredicateCondition.java:68)
	at org.janusgraph.graphdb.query.condition.And.evaluate(And.java:55)
	at org.janusgraph.graphdb.query.graph.GraphCentricQuery.matches(GraphCentricQuery.java:153)
	at com.google.common.collect.Iterators$5.computeNext(Iterators.java:637)
	at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:141)
	at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:136)
	at org.janusgraph.graphdb.query.QueryProcessor.getUnfoldedIterator(QueryProcessor.java:98)
	at org.janusgraph.graphdb.query.QueryProcessor.iterator(QueryProcessor.java:67)
	at org.janusgraph.graphdb.query.graph.GraphCentricQueryBuilder$1.iterator(GraphCentricQueryBuilder.java:204)
	at org.janusgraph.graphdb.query.graph.GraphCentricQueryBuilder$1.iterator(GraphCentricQueryBuilder.java:201)
	at org.janusgraph.graphdb.tinkerpop.optimize.JanusGraphStep.executeGraphCentricQuery(JanusGraphStep.java:160)
	at org.janusgraph.graphdb.tinkerpop.optimize.JanusGraphStep.lambda$new$1(JanusGraphStep.java:95)
	at java.lang.Iterable.forEach(Iterable.java:75)
	at org.janusgraph.graphdb.tinkerpop.optimize.JanusGraphStep.lambda$new$2(JanusGraphStep.java:95)
	at org.apache.tinkerpop.gremlin.process.traversal.step.map.GraphStep.processNextStart(GraphStep.java:157)
	at org.apache.tinkerpop.gremlin.process.traversal.step.util.AbstractStep.hasNext(AbstractStep.java:144)
	at org.apache.tinkerpop.gremlin.process.traversal.util.DefaultTraversal.hasNext(DefaultTraversal.java:196)
	at org.apache.tinkerpop.gremlin.process.traversal.Traversal.tryNext(Traversal.java:97)