daysBetween : exception raised

I need to remove duplicate statistics so I compare two date fields. For some unknown reason "daysBetween" raises an exeption. d1 and d2 are created as local variables (objects also fail). Should I sanitse date fields before a call to daysBetween?

daysBetween($d1,$d2)
^
    at TIK.IVK_compress_new (CreateOrChangeVariable : 'Create Decimal variable')

Advanced stacktrace:
    at com.mendix.languages.mxexpressions.MxExpression.evaluate(MxExpression.scala:15)

Caused by: com.mendix.languages.expressions.ExpressionException: java.lang.NullPointerException
    at com.mendix.languages.expressions.Expr.evaluate(Expr.java:30)

Caused by: java.lang.NullPointerException: null