"the shadow of a doubt" or "a shadow of a doubt" or "shadow of doubt"

Question

It seems all these three versions are in use: "the shadow of a doubt", "a shadow of a doubt", and "shadow of doubt", as suggested by Google Dictionary and Google search results. Google Dictionary (partially from the ODO):

shadow
noun.
the slightest trace of something.
"she knew without a shadow of a doubt that he was lying"
"she knew without any shadow of doubt"

I have always thought "doubt" should follow an article, but strangely the article-less version seems more popular according to Ngram. Any difference in meaning? Or is this a BrE/AmE difference?

Google Ngram:

Answer 1

I'm British and had never thought about this. They all seem equally correct to me as would:

without any shadow of doubt

without the slightest shadow of a doubt

beyond a shadow of a doubt

At this point the whole expression ceases to look meaningful to me so I'll stop.