diff --git a/xen/arch/x86/include/asm/alternative.h b/xen/arch/x86/include/asm/alternative.h
index fa04481316..b54a970e70 100644
--- a/xen/arch/x86/include/asm/alternative.h
+++ b/xen/arch/x86/include/asm/alternative.h
@@ -185,10 +185,10 @@ extern void alternative_branches(void);
  */
 #define ALT_CALL_ARG(arg, n)                                            \
     register union {                                                    \
-        typeof(arg) e;                                                  \
+        typeof(arg) e[sizeof(long) / sizeof(arg)];                      \
         unsigned long r;                                                \
     } a ## n ## _ asm ( ALT_CALL_arg ## n ) = {                         \
-        .e = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); })   \
+        .e[0] = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); })\
     }
 #else
 #define ALT_CALL_ARG(arg, n) \