diff --git a/pkgs/build-support/vm/default.nix b/pkgs/build-support/vm/default.nix
index 1a54723726b..fac85c162e8 100644
--- a/pkgs/build-support/vm/default.nix
+++ b/pkgs/build-support/vm/default.nix
@@ -532,7 +532,7 @@ rec {
 
         rm /mnt/.debug
 
-        ${utillinux}/bin/umount /mnt/nix/store /mnt/tmp ${lib.optionalString unifiedSystemDir /mnt/proc}
+        ${utillinux}/bin/umount /mnt/nix/store /mnt/tmp ${lib.optionalString unifiedSystemDir "/mnt/proc"}
         ${utillinux}/bin/umount /mnt
       '';