diff --git a/include/boost/math/tools/rational.hpp b/include/boost/math/tools/rational.hpp
index 0a3a6d180..467f32af0 100644
--- a/include/boost/math/tools/rational.hpp
+++ b/include/boost/math/tools/rational.hpp
@@ -12,6 +12,7 @@
 
 #include <boost/array.hpp>
 #include <boost/math/tools/config.hpp>
+#include <boost/math/tools/assert.hpp>
 
 #if BOOST_MATH_POLY_METHOD == 1
 #  define BOOST_HEADER() <BOOST_JOIN(boost/math/tools/detail/polynomial_horner1_, BOOST_MATH_MAX_POLY_ORDER).hpp>