Is the rapid radio variability seen in PKS 0537-441 due to microlensing?

BL Lacertae (BL Lac) objects are a population of active galaxies that possess featureless spectra, rapid variability from the X-ray to the radio, and strongly varying polarization. One explanation of these extreme properties invokes the action of gravitational microlensing due to stars in an intervening galaxy, amplifying the continuum of a background active galaxy to swamp the line emission. The BL Lac object PKS 0537 - 441 exhibits rapid variability at radio frequencies, and it has been suggested that microlensing by subsolar-mass stars is a possible source of these fluctuations. In this paper we demonstrate, when considering the physical scale of the optical-through-radio emission region in BL Lac object systems, that microlensing cannot have a dominant influence on the properties of this system, and we therefore suggest that the observed variability is due to some other process.