diff --git a/xen/arch/x86/x86_64/compat/entry.S b/xen/arch/x86/x86_64/compat/entry.S
index e80c53cc71..1b6a482b9c 100644
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -456,7 +456,7 @@ ENTRY(compat_hypercall_table)
         .quad compat_update_va_mapping_otherdomain
         .quad compat_iret
         .quad compat_vcpu_op
-        .quad compat_ni_hypercall       /* 25 */
+        .quad do_set_segment_base       /* 25 */
         .quad compat_mmuext_op
         .quad compat_xsm_op
         .quad compat_nmi_op
diff --git a/xen/arch/x86/x86_64/mm.c b/xen/arch/x86/x86_64/mm.c
index 7f858fb2fc..250d3e0025 100644
--- a/xen/arch/x86/x86_64/mm.c
+++ b/xen/arch/x86/x86_64/mm.c
@@ -1031,6 +1031,9 @@ long do_set_segment_base(unsigned int which, unsigned long base)
     struct vcpu *v = current;
     long ret = 0;
 
+    if ( is_pv_32bit_vcpu(v) )
+        return -ENOSYS; /* x86/64 only. */
+
     switch ( which )
     {
     case SEGBASE_FS: