diff --git a/src/lisp.h b/src/lisp.h
index 8a1b1acf5f0..d4331b3a4ff 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -45,7 +45,7 @@ #define EMACS_LISP_H
 INLINE_HEADER_BEGIN
 
 #ifdef HAVE_MPS
-union gc_header { uint64_t v; };
+union gc_header;
 extern void gc_maybe_quit (void);
 extern bool gc_signal_handler_can_run (int);
 #else
@@ -335,6 +335,7 @@ #define GCALIGNED_UNION_MEMBER char alignas (GCALIGNMENT) gcaligned;
 #endif
 #define GCALIGNED(type) (alignof (type) % GCALIGNMENT == 0)
 
+union gc_header { uint64_t v; GCALIGNED_UNION_MEMBER };
 /* Lisp_Word is a scalar word suitable for holding a tagged pointer or
    integer.  Usually it is a pointer to a deliberately-incomplete type
    'struct Lisp_X'.  However, it is EMACS_INT when Lisp_Objects and
@@ -6335,3 +6336,4 @@ define_error (Lisp_Object name, const char *message, Lisp_Object parent);
 INLINE_HEADER_END
 
 #endif /* EMACS_LISP_H */
+