Calibration problem for VV

i tried to calibrate a dual pol data into beta nought
when converting alone for VH it worked fine, but when trying for VV it shows some error:

org.esa.snap.framework.gpf.OperatorException: Calibration: java.lang.NullPointerException
at org.esa.snap.gpf.OperatorUtils.catchOperatorException(OperatorUtils.java:412)
at org.esa.s1tbx.calibration.gpf.CalibrationOp.computeTile(CalibrationOp.java:184)
at org.esa.snap.framework.gpf.internal.OperatorImage.computeRect(OperatorImage.java:80)
at javax.media.jai.SourcelessOpImage.computeTile(SourcelessOpImage.java:137)
at com.sun.media.jai.util.SunTileScheduler.scheduleTile(SunTileScheduler.java:904)
Caused: org.esa.snap.framework.gpf.OperatorException: Calibration: java.lang.NullPointerException
at org.esa.snap.framework.gpf.internal.OperatorExecutor$GPFImagingListener.errorOccurred(OperatorExecutor.java:374)
at com.sun.media.jai.util.SunTileScheduler.sendExceptionToListener(SunTileScheduler.java:1646)
at com.sun.media.jai.util.SunTileScheduler.scheduleTile(SunTileScheduler.java:921)
at javax.media.jai.OpImage.getTile(OpImage.java:1129)
at javax.media.jai.PlanarImage.getData(PlanarImage.java:2085)
at com.bc.ceres.glevel.MultiLevelImage.getData(MultiLevelImage.java:64)
at org.esa.snap.framework.gpf.internal.OperatorContext.getSourceTile(OperatorContext.java:412)
at org.esa.snap.framework.gpf.internal.OperatorContext.getSourceTile(OperatorContext.java:398)
at org.esa.snap.framework.gpf.internal.OperatorImage.computeRect(OperatorImage.java:73)
at javax.media.jai.SourcelessOpImage.computeTile(SourcelessOpImage.java:137)
at com.sun.media.jai.util.SunTileScheduler.scheduleTile(SunTileScheduler.java:904)
at javax.media.jai.OpImage.getTile(OpImage.java:1129)
at com.sun.media.jai.util.RequestJob.compute(SunTileScheduler.java:247)
[catch] at com.sun.media.jai.util.WorkerThread.run(SunTileScheduler.java:468)