ellap: I can't reproduce example from manual

[Shevek <shevek@anarres.info>]

I'm trying to understand the use of 'ellap' to compute the Frobenius
trace of elliptic curves defined over different fields.

Then I stucked on an example verbatim taken from the manual (*):

---------------------
? Fq = ffgen(ffinit(11,3), 'a); \\ defines Fq := F11^3
? E = ellinit([a+1,a], Fq);  \\ y^2 = x^3 + (a+1)x + a, defined over Fq
? ellap(E)
  ***   at top-level: ellap(E)
  ***                 ^--------
  *** ellap: incorrect type in ellap [can't determine p] (t_VEC).
  ***   Break loop: type 'break' to go back to GP prompt
---------------------

What is wrong here? The example was played on versions 2.9.3 (self
compiled) and 2.7.5 (from Ubuntu package). The system is Ubuntu 16.04

TIA

(*) - http://pari.math.u-bordeaux.fr/dochtml/html/Elliptic_curves.html

-- 
Shevek (shevek at anarres.info)
https://crypto.anarres.info