diff --git a/library/poly1305.c b/library/poly1305.c
index 069b82d..9fc79a9 100644
--- a/library/poly1305.c
+++ b/library/poly1305.c
@@ -509,6 +509,9 @@
     }
 };
 
+/* Make sure no other definition is already present. */
+#undef ASSERT
+
 #define ASSERT( cond, args )            \
     do                                  \
     {                                   \