diff --git a/expat/lib/expat_external.h b/expat/lib/expat_external.h
index aa08a2f8..892eb4bb 100644
--- a/expat/lib/expat_external.h
+++ b/expat/lib/expat_external.h
@@ -93,7 +93,10 @@ extern "C" {
 #endif
 
 #ifdef XML_UNICODE_WCHAR_T
-#define XML_UNICODE
+# define XML_UNICODE
+# if defined(__SIZEOF_WCHAR_T__) && (__SIZEOF_WCHAR_T__ != 2)
+#  error "sizeof(wchar_t) != 2; Need -fshort-wchar for both Expat and libc"
+# endif
 #endif
 
 #ifdef XML_UNICODE     /* Information is UTF-16 encoded. */