diff --git a/include/matrix.hpp b/include/matrix.hpp
index 992c724..ef0c597 100644
--- a/include/matrix.hpp
+++ b/include/matrix.hpp
@@ -11,6 +11,7 @@
 #define __MATRIX__
 
 #include <armadillo>
+#include <omp.h>
 
 /** @brief Create a tridiagonal matrix.
  *