diff --git a/src/systems/si-hep/include/units/isq/si/hep/momentum.h b/src/systems/si-hep/include/units/isq/si/hep/momentum.h
index 21dc44c6..9b53b4c2 100644
--- a/src/systems/si-hep/include/units/isq/si/hep/momentum.h
+++ b/src/systems/si-hep/include/units/isq/si/hep/momentum.h
@@ -34,6 +34,11 @@
 
 namespace units::isq::si::hep {
 
+
+// Necessary to factor `5'344'285'992'678`, which appears in the value for eV/c.
+template<>
+inline constexpr std::optional<std::intmax_t> units::known_first_factor<296'904'777'371> = 157'667;
+
 struct kilogram_metre_per_second : derived_unit<kilogram_metre_per_second> {};
 
 struct eV_per_c :