  Recently introduced connections between quantum codes and Narain CFTs provide
a simple ansatz to express a modular-invariant function $Z(\tau,\bar \tau)$ in
terms of a multivariate polynomial satisfying certain additional properties.
These properties include algebraic identities, which ensure modular invariance
of $Z(\tau,\bar \tau)$, and positivity and integrality of coefficients, which
imply positivity and integrality of the $\mathfrak{u}(1)^n \times
\mathfrak{u}(1)^n$ character expansion of $Z(\tau,\bar \tau)$. Such polynomials
come naturally from codes, in the sense that each code of a certain type gives
rise to the so-called enumerator polynomial, which automatically satisfies all
necessary properties, while the resulting $Z(\tau,\bar \tau)$ is the partition
function of the code CFT -- the Narain theory unambiguously constructed from
the code. Yet there are also ``fake'' polynomials satisfying all necessary
properties, that are not associated with any code. They lead to $Z(\tau,\bar
\tau)$ satisfying all modular bootstrap constraints (modular invariance and
positivity and integrality of character expansion), but whether they are
partition functions of any actual CFT is unclear. We consider the group of the
six simplest fake polynomials and denounce the corresponding $Z$'s as fake: we
show that none of them is the torus partition function of any Narain theory.
Moreover, four of them are not partition functions of any unitary 2d CFT; our
analysis for other two is inconclusive. Our findings point to an obvious
limitation of the modular bootstrap approach: not every solution of the full
set of torus modular bootstrap constraints is due to an actual CFT. In the
paper we consider six simple examples, keeping in mind that thousands more can
be constructed.
