JDK7 changes

I had a surprise this morning while playing with System.getProperties. I am using JDK7 and displaying the system properties in a Swing JTable.
When I don't specify the number of columns I get a null pointer exception. It happens when populating the array contents, not at initialization.
This will throw a NPE:

data = new Object[NB_ROWS][];

This will not

 data = new Object[NB_ROWS][NB_COLUMNS];

Code excerpt

public JavaEnvironmentModel()
        {
            Properties envProperties = System.getProperties();
            NB_ROWS = envProperties.size();
            data = new Object[NB_ROWS][NB_COLUMNS];
 
            // for google blogger parser, no generics(Entry)
            Iterator it = envProperties.entrySet().iterator();

            for (int i = 0; it.hasNext(); i++)
            {
                Entry entry = (Entry)it.next();
                data[i][PROPERTY_COLUMN] = entry.getKey();
                data[i][VALUE_COLUMN] = entry.getValue();
            }
        }