nan.00 in transfers (1.2.X)

[Luke <..hidden..>]

While entering a transfer in 1.2.18 (cash -> transfer):

Date: future dated (by a few days)
asset account: credit 15.51
asset account: debit 15.51
no FX; entered source and memo on each.
update.

Results:

#### asset account    ____   15.51  source memo
#### asset account    15.51   NaN.00  source memo

Why we got a NaN.00 in the credit field for the second asset account?:)

Fortunately, removing it and updating does not cause it to come back.

Still, though, what might have caused it to appear in the first place?

Luke