# R2RML VirtualGraphIterator
**URL:** https://community.stardog.com/t/r2rml-virtualgraphiterator/742
**Category:** Bug
**Created:** [December 21, 2017, 9:39am UTC](https://community.stardog.com/t/r2rml-virtualgraphiterator/742 "2017-12-21T09:39:08Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@timmisset](https://community.stardog.com/u/timmisset)
#### Post date: [December 21, 2017, 9:39am UTC](https://community.stardog.com/t/r2rml-virtualgraphiterator/742/1 "2017-12-21T09:39:08Z")
I receive a nullpointer exception when I run virtual import on a SQLServer database. Using the exact same configuration I can run virtual add and query all the data without any problems.
I run both commands using --format r2rml
I actually discovered what caused the behavior. One of the columns used to create a predicateobject map was called [column] (its a grid position so that name made sense and by using [column] instead of column this works fine in T-SQL and apparently also in the Virtual Add interpreter. I changed the alias to \_column and now it can be parsed without problems by the Virtual Import.
java.lang.NullPointerException
at com.complexible.stardog.virtual.VirtualGraphIterator$MaterializationContext.getColumnIndex(VirtualGraphIterator.java:268)
at com.complexible.stardog.virtual.ValueGeneratorFactory.getTermString(ValueGeneratorFactory.java:109)
at com.complexible.stardog.virtual.ValueGeneratorFactory.createValueGenerator(ValueGeneratorFactory.java:47)
at com.complexible.stardog.virtual.VirtualGraphIterator.createValueGenerator(VirtualGraphIterator.java:198)
at com.complexible.stardog.virtual.VirtualGraphIterator.nextMapping(VirtualGraphIterator.java:143)
at com.complexible.stardog.virtual.VirtualGraphIterator.computeNext(VirtualGraphIterator.java:209)
at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:143)
at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:138)
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### Author:  [@stephen](https://community.stardog.com/u/stephen)
#### Post date: [December 21, 2017, 5:04pm UTC](https://community.stardog.com/t/r2rml-virtualgraphiterator/742/2 "2017-12-21T17:04:40Z")
> [@timmisset](#):
>
> I actually discovered what caused the behavior. One of the columns used to create a predicateobject map was called [column] (its a grid position so that name made sense and by using [column] instead of column this works fine in T-SQL and apparently also in the Virtual Add interpreter. I changed the alias to \_column and now it can be parsed without problems by the Virtual Import.
Quoting the answer since it's sort of hidden within the question.
Thanks for the report; we'll look into making this better. Glad you were able to figure it out!
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### Author:  [@jess](https://community.stardog.com/u/jess)
#### Post date: [December 21, 2017, 9:31pm UTC](https://community.stardog.com/t/r2rml-virtualgraphiterator/742/3 "2017-12-21T21:31:30Z")
Thanks for the bug report. Can you provide a snippet of the query? Which version of Stardog are you using? I’m unable to reproduce the error.
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### Author:  [@timmisset](https://community.stardog.com/u/timmisset)
#### Post date: [December 22, 2017, 10:20am UTC](https://community.stardog.com/t/r2rml-virtualgraphiterator/742/4 "2017-12-22T10:20:01Z")
I’m using 5.0.2
Line(s) that caused the error:
rr:predicateObjectMap [rr:predicate arrayItem:hasColumn; rr:objectMap [rr:column “[column]”]; ];
rr:predicateObjectMap [rr:predicate arrayItem:hasRow; rr:objectMap [rr:column “[row]”]; ];
Fixed by:
rr:predicateObjectMap [rr:predicate arrayItem:hasColumn; rr:objectMap [rr:column “\_column”]; ];
rr:predicateObjectMap [rr:predicate arrayItem:hasRow; rr:objectMap [rr:column “\_row”]; ];
And of course by changing the SQL view to output \_column and \_row