  We present low-radio-frequency follow-up observations of AT 2017gfo, the
electromagnetic counterpart of GW170817, which was the first binary neutron
star merger to be detected by Advanced LIGO-Virgo. These data, with a central
frequency of 144 MHz, were obtained with LOFAR, the Low-Frequency Array. The
maximum elevation of the target is just 13.7 degrees when observed with LOFAR,
making our observations particularly challenging to calibrate and significantly
limiting the achievable sensitivity. On time-scales of 130-138 and 371-374 days
after the merger event, we obtain 3$\sigma$ upper limits for the afterglow
component of 6.6 and 19.5 mJy beam$^{-1}$, respectively. Using our best upper
limit and previously published, contemporaneous higher-frequency radio data, we
place a limit on any potential steepening of the radio spectrum between 610 and
144 MHz: the two-point spectral index $\alpha^{610}_{144} \gtrsim -2.5$. We
also show that LOFAR can detect the afterglows of future binary neutron star
merger events occurring at more favourable elevations.
