Using capitalize in a template

[mattiasb]

This template doesn't work as expected:

(example ";;" (capitalize str)
         n (s str))

If I expand it in an empty buffer and input for example "testing testing" for
the s I would expect the following to be printed in my buffer:

;; Testing testing
testing testing

Instead I get:

testing testing;;
︱

It's probably easier with a screen shot:

It looks like there's some sort of marker where the (s str) was supposed
to be input but the input happened at the beginning of the template instead. Also the capitalized string wasn't input at all.

Other similar templates do work:

(example2 ";;" str
          n (s str))

↓↓↓↓↓↓↓↓↓↓

;;testing testing
testing testing

(example3 ";;" (make-string (length str) ?-)
          n (s str))

↓↓↓↓↓↓↓↓↓↓

;;---------------
testing testing

Is there something special going on with capitalize in particular? I see that it's implemented in C. Does that matter somehow?

[mattiasb]

If I use (p "" str) over (s str) the template works fine.

Wild guess: The s element will bind NAME in (str NAME) to
nil instead of "" when the field is empty. This means that
(capitalize str) in my template will error out before I get to
type anything.

If that's the case the following template should work:

(example ";;" (capitalize (or str ""))
         n (s str))

And the reason the make-string calls in example2 works is
because nil is the empty list so (length nil) is 0 and not
a failure.

And now I've just tested this and it's indeed the case.

Now, I can't think of a case where I would want (s NAME) to
bind NAME to nil over "" when the field is empty and it's
a source of confusion and bugs.

Should I file a bug for this?