diff --git a/opengl/tests/angeles/demo.c b/opengl/tests/angeles/demo.c
index 802f398..39d871e 100644
--- a/opengl/tests/angeles/demo.c
+++ b/opengl/tests/angeles/demo.c
@@ -666,7 +666,7 @@
         y[2] /= mag;
     }
 
-#define M(row,col)  m[col*4+row]
+#define M(row,col)  m[(col)*4+(row)]
     M(0, 0) = x[0];
     M(0, 1) = x[1];
     M(0, 2) = x[2];