  Recent data on photoproduction of $\eta'$ mesons off protons have been
included in the data base for the Bonn-Gatchina partial wave analysis. The real
and imaginary parts of the $S$-wave $\eta^\prime p\to \eta^\prime p$ scattering
amplitude in the threshold region were fit to yield the $\eta^\prime p$
scattering length and the interaction range. This new analysis found
$|a_{\eta^\prime p}|= (0.403\pm 0.015\pm 0.060)$ fm and a phase
$\phi=(87\pm2)^\circ$, while the range parameter is not well-constrained. The
striking behavior of the GRAAL data on the beam asymmetry in the threshold
region suggests that a narrow proton-$\eta'$ resonance might exist. However,
the scattering length was found to be relatively insensitive to the possible
existence of this narrow resonance.
