  We show that a blazar classification in BL Lacs and Flat Spectrum Radio
Quasars may not be adequate when it relies solely on the equivalent widths
(EWs) of optical lines. In fact, depending on redshift, some strong emission
lines can fall in the infrared window and be missed. We selected a sample of BL
Lacs with firm redshift identification and good visibility from Paranal. We
targeted with the X-shooter spectrograph the five BL Lacs with z > 0.7, i.e.,
those for which the Halpha line, one of the strongest among blazars, falls
outside the optical window and determined the EW of emission lines in both the
infrared and optical bands. Two out of five sources show an observed Halpha EW
> 5A (one has rest frame EW > 5A) and could be classified as FSRQs by one of
the classification schemes used in the literature. A third object is
border-line with an observed EW of 4.4 +/- 0.5A. In all these cases Halpha is
the strongest emission line detected. The Halpha line of the other two blazars
is not detected, but in one case it falls in a region strongly contaminated by
sky lines and in the other one the spectrum is featureless. We conclude that a
blazar classification based on EW width only can be inaccurate and may lead to
an erroneous determination of blazar evolution. This effect is more severe for
the BL Lac class, since FSRQs can be misclassified as BL Lacs especially at
high redshifts (z > 0.7), where the latter are extremely rare.
