AutoCAD drawings may contain hatches which - mostly due to the loss of associativity - do not have the "Area" property and so you cannot use them to summarize the total area. For area calculations you will need to fix them first - reconstruct their boundary curve and re-create the hatch.
To identify such complaining hatches, we made a LISP utility SelHatch0 - it selects all hatches that do not contain the property Area - so e.g. the Properties windows doesn't display the area of such hatch object.
You can download the SelHatch0 utility from Download, laod it with APPLOAD and run it with the command SELHATCH0. The complaining hatches will be pre-selected for editing.
If you only have AutoCAD LT and cannot use the LISP utility, the selection filtering will help. Use the QSELECT command, use the "Select Objects" button to select all objects in the drawing (by window), select Hatches as the Object Type, select the Area property, and as a criteria, specify "Is greater than" value 0. Click "Remove from new selection set". After confirming the dialog, you will be left with the selected hatches that do not show Area.