diff --git a/lagarto-ox.nix b/lagarto-ox.nix
index 1eddbdc..0a96131 100644
--- a/lagarto-ox.nix
+++ b/lagarto-ox.nix
@@ -86,7 +86,7 @@
       #   -l 4 /dev/qdma34000-MM-1; sleep 0.2; done
       preDeviceCommands = ''
         echo "Creating a heartbeat counter at 0x1bfff0000"
-        hb=0; while [ 1 ]; do let hb=$hb+1; devmem 0x1bfff0000 32 $hb; done &
+        sh -c 'hb=0; while [ 1 ]; do let hb=$hb+1; devmem 0x1bfff0000 32 $hb; done' &
       '';
     };