diff --git a/s/x86_64.ss b/s/x86_64.ss
index 91f6d5f9fb..c62724fb73 100644
--- a/s/x86_64.ss
+++ b/s/x86_64.ss
@@ -43,7 +43,7 @@
       [%Cfparg2            #f  1 fp]
       [%fptmp1             #f  4 fp]  ; xmm 0-5 are caller-save
       [%fptmp2             #f  5 fp]  ; xmm 6-15 are callee-save
-      [%sp                 #t  4 fp])
+      [%sp                 #t  4 uptr])
     (reify-support %ts))
   (define-registers
     (reserved