One potential complication is the physical size of the star itself, which projects to an angular size of \[\theta_*=R/D_{\rm L}=4.65 \mbox{mas} \frac{R/R_\odot}{D_{\rm L}/pc}\] Using the rough rule of thumb that \(R/R_\odot \approx M/M_\odot\), the angular size of the star thus equal to its Einstein radius at a distance of \[D_{\rm L} = 5.7\times10^{-4} \mbox{pc} (M/M_\odot)\] So we see that all dwarf stars except the Sun have Einstein radii larger than their angular radii. Brown dwarfs have size \(\sim 0.1 R_\odot\), which even at 2.5 pc yields \(\sim\)0.2 mas, much smaller than its Einstein ring.