diff --git a/src/matrix-math.c b/src/matrix-math.c
index e6f2130..03895b5 100644
--- a/src/matrix-math.c
+++ b/src/matrix-math.c
@@ -1,6 +1,7 @@
 #include "matrix-math.h"
 #include <math.h>
 #include <stdio.h>
+#include <string.h>
 
 /**
  * overwrites a 4x4 matrix with the identity matrix