We use the continuity equation to derive a method for measuring the pattern speed of the Milky Way's bar/bulge from proper motion data. The method has minimal assumptions but requires complete coverage of the non-axisymmetric component in two of the three Galactic coordinates. We apply our method to the proper motion data from a combination of Gaia DR2 and VISTA Variables in the Via Lactea (VVV) to measure the pattern speed of the bar as $\Omega p=(41\pm 3)\mathrm{km}s-1\mathrm{kpc}-1$ (where the error is statistical). This puts the corotation radius at $(5.7\pm 0.4)\mathrm{kpc}$, under the assumptions of the standard peculiar motion of the Sun and the absence of non-axisymmetric streaming in the Solar neighbourhood. The obtained result uses only data on the near-side of the bar which produces consistent measurements of the distance and velocity of the centre of the Galaxy. Addition of the data on the far-side of the bar pulls the pattern speed down to $\Omega p=(31\pm 1)\mathrm{km}s-1\mathrm{kpc}-1$ but requires a lower transverse velocity for the Galactic centre than observed. This suggests systematics of $5-10\mathrm{km}s-1\mathrm{kpc}-1$ dominate the uncertainty. We demonstrate using a dynamically-formed bar/bulge simulation that even with the limited field of view of the VVV survey our method robustly recovers the pattern speed.