  We measured the degree of linear polarization P of comet C/2018 V1
(Machholz-Fujikawa-Iwamoto) with the broadband Johnson V filter in mid-November
of 2018. Within a radius of \r{ho}=17,000 km of the inner coma, we detected an
extremely low linear polarization at phase angles from 83 to 91.2 degree and
constrained the polarization maximum to Pmax = (6.8 +/- 1.8)%. This is the
lowest Pmax ever measured in a comet. Using model agglomerated debris
particles, we reproduced the polarimetric response of comet C/2018 V1. Four
retrieved refractive indices closely match what was experimentally found in
Mg-rich silicates with little or no iron content. Moreover, the size
distribution of the agglomerated debris particles appears in good quantitative
agreement with the in situ findings of comet 1P/Halley. The dust model of
polarization of comet C/2018 V1 suggests a strongly negative polarization with
amplitude |Pmin| = 5%-7%; whereas, an interpretation based on gaseous emission
requires no negative polarization at small phase angles. This dramatic
difference could be used to discriminate gaseous-emission and dust explanations
in low-Pmax comets in future.
