diff --git a/include/os.h b/include/os.h
index 166c60cd8..276eb5213 100644
--- a/include/os.h
+++ b/include/os.h
@@ -321,7 +321,8 @@ extern _X_EXPORT void
 OsCleanup(Bool);
 
 extern _X_EXPORT void
-OsVendorFatalError(const char *f, va_list args);
+OsVendorFatalError(const char *f, va_list args)
+_X_ATTRIBUTE_PRINTF(1, 0);
 
 extern _X_EXPORT void
 OsVendorInit(void);