Strange text ('Unk') being displayed as label

[Xin]

I am using a data that represents urban areas.  I display the label field
when the area is of a certain minimal size.  This works great,  but on
closer inspection I have noticed 'Unk' being displayed at places.  This
doesn't happen often, but often enough to be a concern.

I links below are images showing this.

http://img.auctiva.com/imgdata/0/1/6/7/2/6/webimg/5867510_o.jpg

http://img.auctiva.com/imgdata/0/1/6/7/2/6/webimg/5867511_o.jpg

I have checked in the data, and there is only one area called 'Unk'.

Below is the mapfile code for that particular layer.

> LAYER
>     # Urban Densitiy
>     NAME "BUILTUPA"
>     STATUS DEFAULT
>     DATA "BUILTUPA_AREA"
>     TYPE POLYGON
>     MAXSCALE 6000000
>     LABELITEM "NAM"
>
>     # Big Cities
>     CLASS
>         EXPRESSION ([AREA_SQMI] > 40)
>         MINSCALE 250000
>       LABEL
>         TYPE TRUETYPE
>         FONT "verdana_bold"
>         SIZE 12
>         POSITION AUTO
>         ANGLE AUTO
>         BUFFER 2
>         MINDISTANCE 10
>         COLOR 40 40 40
>         OUTLINECOLOR 240 240 240
>         ANTIALIAS TRUE
>         PARTIALS TRUE
>                 MINFEATURESIZE 8
>       END
>       STYLE
>         COLOR 210 210 210
>       END
>     END
>
>     # Cities
>     CLASS
>         MAXSCALE 2600000
>         EXPRESSION ([AREA_SQMI] > 25)
>       LABEL
>         TYPE TRUETYPE
>         FONT "verdana"
>         SIZE 12
>         POSITION AUTO
>         ANGLE AUTO
>         BUFFER 2
>         MINDISTANCE 10
>         COLOR 40 40 40
>         OUTLINECOLOR 240 240 240
>         ANTIALIAS TRUE
>         PARTIALS TRUE
>                 MINFEATURESIZE 8
>       END
>       STYLE
>         COLOR 210 210 210
>       END
>     END
>
>      # Towns
>     CLASS
>         MAXSCALE 800000
>         EXPRESSION ([AREA_SQMI] > 5 and [AREA_SQMI] < 40)
>       LABEL
>         TYPE TRUETYPE
>         FONT "verdana"
>         SIZE 11
>         POSITION AUTO
>         ANGLE AUTO
>         BUFFER 2
>         MINDISTANCE 10
>         COLOR 40 40 40
>         OUTLINECOLOR 240 240 240
>         ANTIALIAS TRUE
>         PARTIALS TRUE
>                 MINFEATURESIZE 8
>       END
>       STYLE
>         COLOR 210 210 210
>       END
>     END
>     PROJECTION
>         "proj=latlong"
>     END
>   END
>

Any help is greatly appreciated.

Xin
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