DateTimeAxis throws Error #1009 when there is null date

Steps to reproduce:
1. Run test.mxml.

Actual Results:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at mx.charts::DateTimeAxis/mapCache()[C:\Work\flex\dmv_automation\projects\datavisualisation\src\mx\charts\DateTimeAxis.as:1403]
at mx.charts.series::ColumnSeries/updateMapping()[C:\Work\flex\dmv_automation\projects\datavisualisation\src\mx\charts\series\ColumnSeries.as:1184]

Expected Results:
Null date does not appear in the chart.

Workaround (if any):
I extend DateTimeAxis.mapCache (see DTAxis.as) and in test.mxml replace:
<mx:DateTimeAxis dataUnits="days" />
By
<local:DTAxis dataUnits="days" />

The problem happens in lines:
1386: v[convertedField] = Date.parse(v[field].toString());
1394: v[convertedField] = Number(v[field].toString());
1403: v[convertedField] = v[field].getTime();
because v[field] may be null.