diff --git a/src/csg/matrix_functions.cpp b/src/csg/matrix_functions.cpp
index 429471679832911831f3545844411cdce376b777..dafd08f7056af87525f6d21d659b1f242fac7179 100644
--- a/src/csg/matrix_functions.cpp
+++ b/src/csg/matrix_functions.cpp
@@ -2,7 +2,7 @@
 
 namespace {
 double determinant(const matrix::Mat2d &m) {
-  return m[0][0] * m[1][1] - m[1][0] * m[1][1];
+  return m[0][0] * m[1][1] - m[1][0] * m[0][1];
 }
 
 double determinant(const matrix::Mat3d &m) {