diff --git a/library/chacha20.c b/library/chacha20.c
index bda39b2..d8a51ec 100644
--- a/library/chacha20.c
+++ b/library/chacha20.c
@@ -516,6 +516,9 @@
     375U
 };
 
+/* Make sure no other definition is already present. */
+#undef ASSERT
+
 #define ASSERT( cond, args )            \
     do                                  \
     {                                   \