  High synchrotron peak (HSP; $\nu_{sy}^{pk} > 10^{15}$ Hz) BL Lacs are some of
the most extreme accelerators in the Universe. Those found at high redshifts
($z>1$) challenge our understanding of blazar evolution models and are crucial
for cosmological measurements of the Extragalactic Background Light. In this
paper, we study a high-$z$ BL Lac, 4FGL J2146.5-1344, detected to be at
$z$=1.34 using the photometric dropout technique. We collected multi-wavelength
data for this source from optical up to $\gamma$-rays, in order to study its
spectral energy distribution (SED). In particular, this source was observed for
the first time with NuSTAR, which accurately measures the synchrotron emission
of this blazar up to 50 keV. Despite being classified as an HSP BL Lac object,
the modeling of the SED reveals that this source likely belongs to the
"masquerading BL Lac" class, which comprises of FSRQs appearing as disguised BL
Lac objects.
