diff --git a/lib/libdisk/libdisk.3 b/lib/libdisk/libdisk.3
index e097cba4677..d4139798f1f 100644
--- a/lib/libdisk/libdisk.3
+++ b/lib/libdisk/libdisk.3
@@ -94,7 +94,7 @@
 .Ft char **
 .Fn Disk_Names "void"
 .Ft void
-.Fn Set_Boot_Mgr "struct disk *d" "const u_char *bootmgr"
+.Fn Set_Boot_Mgr "struct disk *d" "const u_char *bootmgr" "const size_t bootmgr_size"
 .Ft void
 .Fn Set_Boot_Blocks "struct disk *d" "const u_char *boot1" "const u_char *boot2"
 .Ft int