diff --git a/include/c3d/maths.h b/include/c3d/maths.h
index e0270ae..f08bb7f 100644
--- a/include/c3d/maths.h
+++ b/include/c3d/maths.h
@@ -273,6 +273,7 @@ void Mtx_Identity(C3D_Mtx* out);
 
 /**
  * @brief Multiply two matrices
+ * @note Undefined behavior if arguments, out, is either a or b.
  * @param[out] out Output matrix
  * @param[in]  a   Multiplicand
  * @param[in]  b   Multiplier