diff --git a/lib/matrix.h b/lib/matrix.h
index a3c264062..4d4af14ed 100644
--- a/lib/matrix.h
+++ b/lib/matrix.h
@@ -50,7 +50,7 @@ inline void MatrixClear(Matrix<fp_type>& m) {
  */
 template <typename fp_type>
 inline void MatrixIdentity(unsigned n, Matrix<fp_type>& m) {
-  m.resize(2 * n * n);
+  m.resize(2 * static_cast<typename Matrix<fp_type>::size_type>(n) * n);
 
   MatrixClear(m);