Unknow datatype error for blank nodes

[patrickwestphal]

When using blank nodes, e.g. using this view definition

Create View err As
  Construct {
    <http://ex.org/res> <http://ex.org/prop> _:23 .
  }

I get this error message:

Exception in thread "main" java.lang.RuntimeException: Unknow datatype of node: _:b0
    at org.aksw.sparqlify.expr.util.NodeValueUtils.getValue(NodeValueUtils.java:66)
    at org.aksw.sparqlify.core.algorithms.MappingOpsImpl.pushConstants(MappingOpsImpl.java:1406)
    at org.aksw.sparqlify.core.algorithms.MappingOpsImpl.pushConstants(MappingOpsImpl.java:1335)
    at org.aksw.sparqlify.core.algorithms.MappingOpsImpl.union(MappingOpsImpl.java:1516)
    at org.aksw.sparqlify.core.algorithms.OpMappingRewriterImpl.rewrite(OpMappingRewriterImpl.java:101)
    at org.aksw.sparqlify.core.algorithms.OpMappingRewriterImpl.rewrite(OpMappingRewriterImpl.java:334)
    at org.aksw.sparqlify.core.interfaces.SparqlSqlOpRewriterImpl.rewrite(SparqlSqlOpRewriterImpl.java:96)
    at org.aksw.sparqlify.core.algorithms.SparqlSqlStringRewriterImpl.rewrite(SparqlSqlStringRewriterImpl.java:45)
    at org.aksw.sparqlify.core.sparql.QueryExecutionSelect._execSelect(QueryExecutionSelect.java:131)
    at org.aksw.sparqlify.core.sparql.QueryExecutionSelect.execSelect(QueryExecutionSelect.java:57)
    at org.aksw.jena_sparql_api.core.QueryExecutionBaseSelect.executeCoreSelect(QueryExecutionBaseSelect.java:146)
    at org.aksw.sparqlify.core.sparql.QueryExecutionSparqlify.executeCoreSelect(QueryExecutionSparqlify.java:182)
    at org.aksw.jena_sparql_api.core.QueryExecutionBaseSelect.executeConstructStreaming(QueryExecutionBaseSelect.java:342)
    at org.aksw.jena_sparql_api.core.QueryExecutionBaseSelect.execConstructTriples(QueryExecutionBaseSelect.java:371)
    at org.aksw.jena_sparql_api.core.utils.QueryExecutionUtils.createIteratorDumpTriples(QueryExecutionUtils.java:65)
    at org.aksw.sparqlify.web.Main.main(Main.java:268)

The same holds if I use the bNode term constructor instead of _.23.
I'm using the 0.6.9-SNAPSHOT version.