.Preconditions.checkNotNull error

Hi, Any idea what causes this error when building a yield curve?

Exception in thread “main” java.lang.NullPointerException
at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:770)
at com.google.common.collect.SingletonImmutableList.(SingletonImmutableList.java:36)
at com.google.common.collect.ImmutableList.of(ImmutableList.java:75)
at com.opengamma.strata.pricer.curve.CurveCalibrator.calibrate(CurveCalibrator.java:208)

The error is due to putting a null in the list, which is not allowed. Without more information, we can’t say what caused it. You’d need to specify what was passed in. You might want to consider using an IDE to debug where the null comes from.