diff --git a/include/wx/chartype.h b/include/wx/chartype.h
index 0e81a3344d..3ff445f602 100644
--- a/include/wx/chartype.h
+++ b/include/wx/chartype.h
@@ -175,6 +175,10 @@
     #define wxUSE_UTF8_LOCALE_ONLY 0
 #endif
 
+#ifndef SIZEOF_WCHAR_T
+    #error "SIZEOF_WCHAR_T must be defined before including this file in wx/defs.h"
+#endif
+
 #if wxUSE_UNICODE_WCHAR && SIZEOF_WCHAR_T == 2
     #define wxUSE_UNICODE_UTF16 1
 #else