This may be a pretty obscure situation, and I have a workaound so its not a big deal, but I came across a situation where during a build this message was generated:
in dict1.txt at 12: +
entering null word to dictionary in sentence
I isolated the cause down to, in a table definition, bursting a string into individual characters where that string contains a space and the result is being explicitly stored in a fact set.
table: a (^name)
@0 = ^burst(^name "")
data:
"some name"