[Therion] Strange scale-dependent Metapost error

[Duncan Collis]

Hi,

I have a survey which compiles correctly at 1:500 or 1:2000, but at
1:1000 produces the following error from Metapost:

This is MetaPost, Version 0.641 (Web2c 7.5.2) (mem=mpost 2004.3.2)  9
MAR 2009 13:06
**data.mp
(data.mp [4001] [4002] [4003] [4004] [4005] [4006] [4007]
[4008] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
[32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46]
[47] [48] [49] [50] [51] [52] [53] [54] [55]
! I can't find file `mptextmp'.
<scantokens> input mptextmp

<to be read again>
                   endgroup
<to be read again>
                   ,
s_scalebar->...mal((EXPR0))&"\thinspace"&(EXPR2)),
                                                  origin);endgroup
l.10077 ...ar(100, 1, "\thfb\char109 \mainfont{}")
                                                  ;
Please type another input file name
! Emergency stop.
<scantokens> input mptextmp

<to be read again>
                   endgroup
<to be read again>
                   ,
s_scalebar->...mal((EXPR0))&"\thinspace"&(EXPR2)),
                                                  origin);endgroup
l.10077 ...ar(100, 1, "\thfb\char109 \mainfont{}")
                                                  ;
*** (job aborted, file error in nonstop mode)




The problem disappears when a particular scrap is commented-out in the
map definition, although I can't see anything particularly strange
about that scrap.

How should I go about debugging this?

Duncan.