diff --git a/src/core/include/units/ratio.h b/src/core/include/units/ratio.h
index 923f2355..caf2571f 100644
--- a/src/core/include/units/ratio.h
+++ b/src/core/include/units/ratio.h
@@ -52,7 +52,7 @@ struct ratio {
   std::intmax_t den;
   std::intmax_t exp;
 
-  constexpr explicit ratio(std::intmax_t n, std::intmax_t d = 1, std::intmax_t e = 0): num(n), den(d), exp(e)
+  constexpr explicit(false) ratio(std::intmax_t n, std::intmax_t d = 1, std::intmax_t e = 0): num(n), den(d), exp(e)
   {
     gsl_Expects(den != 0);
     detail::normalize(num, den, exp);