diff --git a/source/mijin/debug/assert.hpp b/source/mijin/debug/assert.hpp
index 6c0b90f..033515f 100644
--- a/source/mijin/debug/assert.hpp
+++ b/source/mijin/debug/assert.hpp
@@ -108,6 +108,10 @@ if (!static_cast<bool>(condition))                                          \
 // public types
 //
 
+#if defined(ERROR)
+#error "Someone included windows.h! Include mijin/util/winundef.h."
+#endif
+
 enum class AssertionResult
 {
     ABORT = -1,    // call std::abort()