diff --git a/include/xo/ratio/ratio.hpp b/include/xo/ratio/ratio.hpp
index 36087e7e..5496e05e 100644
--- a/include/xo/ratio/ratio.hpp
+++ b/include/xo/ratio/ratio.hpp
@@ -53,7 +53,7 @@ namespace xo {
              *
              *  Ratio need not be normalized
              **/
-            constexpr ratio(Int n, Int d) : num_{n}, den_{d} {}
+            constexpr ratio(Int n, Int d = 1) : num_{n}, den_{d} {}
             ///@}
 
             /** @defgroup ratio-static-methods **/