Jade, Why variables are sent in single quotes in one case and unquoted in another?

I put some data from MongoDB into a Jade template as simple as this:

  span(data-id=job._id)
  span= job._id

But for some reason I get first span int the output HTML markup looking like <span data-id="'5188b71aaeacb0c503000002'"></span> while second span looks better - <span>5188b71aaeacb0c503000002</span>.

Why in first case single quotes are added there?

I have been unable to get two diferent values from job._id .

TEST 1:

passing: {"job": {"_id": "hello"}}

your jade:

span(data-id=job._id)
span= job._id

Html output:

<span data-id="hello"></span>    <-- No extra ''
<span>hello</span>

TEST 2:

passing: {"job": {"_id": "'hello'"}} <-- extra ''

the jade:

span(data-id=job._id)
span #{job._id}       // or  span= job._id

Html output:

<span data-id="'hello'"></span>
<span>'hello'</span>