diff --git a/include/boost/math/interpolators/detail/cardinal_quadratic_b_spline_detail.hpp b/include/boost/math/interpolators/detail/cardinal_quadratic_b_spline_detail.hpp
index 57137f465..cb5ce9f57 100644
--- a/include/boost/math/interpolators/detail/cardinal_quadratic_b_spline_detail.hpp
+++ b/include/boost/math/interpolators/detail/cardinal_quadratic_b_spline_detail.hpp
@@ -154,8 +154,8 @@ public:
         using std::floor;
         using std::ceil;
         Real x = (t-m_t0)*m_inv_h;
-        size_t j_min = ceil(x - Real(1)/Real(2));
-        size_t j_max = ceil(x + Real(5)/Real(2));
+        auto j_min = static_cast<size_t>(ceil(x - Real(1)/Real(2)));
+        auto j_max = static_cast<size_t>(ceil(x + Real(5)/Real(2)));
         if (j_max >= m_alpha.size()) {
             j_max = m_alpha.size() - 1;
         }
@@ -180,8 +180,8 @@ public:
         using std::floor;
         using std::ceil;
         Real x = (t-m_t0)*m_inv_h;
-        size_t j_min = ceil(x - Real(1)/Real(2));
-        size_t j_max = ceil(x + Real(5)/Real(2));
+        auto j_min = static_cast<size_t>(ceil(x - Real(1)/Real(2)));
+        auto j_max = static_cast<size_t>(ceil(x + Real(5)/Real(2)));
         if (j_max >= m_alpha.size()) {
             j_max = m_alpha.size() - 1;
         }