[GNC] Crash in Net worth linechart

[Fred Tydeman]

I am trying to do a Net Worth Linechart for one account (with two
investments) in COP (Colombia Pesos).  This is what I get:

Report error

An error occurred while running the report.

          11 (apply-smob/1 #)
In options.scm:
  2037:10 10 (run-callbacks)
In srfi/srfi-1.scm:
    640:9  9 (for-each # …)
In unknown file:
           8 (gncp-option-invoke-callback # …)
In c-interface.scm:
     35:4  7 (gnc:call-with-error-handling _ _)
In ice-9/boot-9.scm:
    829:9  6 (catch _ _ # …)
In c-interface.scm:
    39:40  5 (_)
In report-core.scm:
   752:25  4 (gnc:report-render-html #< type: "d8b63264186b11e19038…> …)
In standard/net-charts.scm:
   316:30  3 (net-renderer #< type: "d8b63264186b11e190380015582913…> …)
    233:6  2 (process-datelist _ _ _)
In srfi/srfi-1.scm:
   466:18  1 (fold # …)
In standard/net-charts.scm:
    235:9  0 (_ _ 0)

In procedure +: Wrong type: #f

However, if I change Price Source from Weighted Average to Closest to
report date, I get no crash and a plot.
================
If I do the same report, but with a different Colombia brokerage
account, it works just fine.

Each of the two Colombia brokerage accounts holds one money market
account and one COP account.  If I look at the Price database, they
both have reasonable prices; nothing looks bad there.

Aside:  If I enable Show table in the Display Options, I can get the
month end values for each month in question, with each month on its
own line.