diff --git a/library/chachapoly.c b/library/chachapoly.c
index d51227a..b05a145 100644
--- a/library/chachapoly.c
+++ b/library/chachapoly.c
@@ -472,6 +472,9 @@
     }
 };
 
+/* Make sure no other definition is already present. */
+#undef ASSERT
+
 #define ASSERT( cond, args )            \
     do                                  \
     {                                   \