diff --git a/src/include/lean/lean.h b/src/include/lean/lean.h
index fbdc24c661..559a7bc30a 100644
--- a/src/include/lean/lean.h
+++ b/src/include/lean/lean.h
@@ -41,12 +41,14 @@ extern "C" {
 #define LEAN_ALWAYS_INLINE
 #endif
 
+#ifndef assert
 #ifdef NDEBUG
 #define assert(expr)
 #else
 void lean_notify_assert(const char * fileName, int line, const char * condition);
 #define assert(expr) { if (LEAN_UNLIKELY(!(expr))) lean_notify_assert(__FILE__, __LINE__, #expr); }
 #endif
+#endif
 
 #ifdef _WIN32
 #define LEAN_EXPORT __declspec(dllexport)