diff --git a/src/ci_orchestrator/winimage/assets/post-install.win.ps1.tmpl b/src/ci_orchestrator/winimage/assets/post-install.win.ps1.tmpl
index dfe252e..97196f3 100644
--- a/src/ci_orchestrator/winimage/assets/post-install.win.ps1.tmpl
+++ b/src/ci_orchestrator/winimage/assets/post-install.win.ps1.tmpl
@@ -264,7 +264,11 @@ reg add 'HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced' /v La
 L 'Explorer UX tweaks applied (HKCU)'
 
 # ── No desktop wallpaper (keep the WS2025 default background colour) ──────
-reg add 'HKCU\Control Panel\Desktop' /v Wallpaper      /t REG_SZ /d '' /f | Out-Null
+# NOTE: route the empty-value reg add through cmd /c. PowerShell 5.1 STRIPS an
+# empty-string argument ('') when calling a native exe, so `reg add ... /d '' /f`
+# would arrive as `reg add ... /d /f` — reg then eats /f as the data, loses the
+# force flag, and PROMPTS "overwrite? (Yes/No)" on stdin → the script hangs.
+cmd /c 'reg add "HKCU\Control Panel\Desktop" /v Wallpaper /t REG_SZ /d "" /f' | Out-Null
 reg add 'HKCU\Control Panel\Desktop' /v WallpaperStyle /t REG_SZ /d 0  /f | Out-Null
 L 'Wallpaper removed (default background colour, HKCU)'
 
@@ -284,7 +288,8 @@ reg add 'HKU\DefaultUser2\Software\Microsoft\Windows\CurrentVersion\Explorer\Adv
 reg add 'HKU\DefaultUser2\Console\%Startup' /v DelegationConsole  /t REG_SZ /d '{E12CFF52-A866-4C77-9A90-F570A7AA2C6B}' /f | Out-Null
 reg add 'HKU\DefaultUser2\Console\%Startup' /v DelegationTerminal /t REG_SZ /d '{E12CFF52-A866-4C77-9A90-F570A7AA2C6B}' /f | Out-Null
 # No wallpaper (keep default background colour) for any future profile.
-reg add 'HKU\DefaultUser2\Control Panel\Desktop' /v Wallpaper      /t REG_SZ /d '' /f | Out-Null
+# cmd /c for the empty value — see PS-5.1 empty-arg note in the HKCU block above.
+cmd /c 'reg add "HKU\DefaultUser2\Control Panel\Desktop" /v Wallpaper /t REG_SZ /d "" /f' | Out-Null
 reg add 'HKU\DefaultUser2\Control Panel\Desktop' /v WallpaperStyle /t REG_SZ /d 0  /f | Out-Null
 reg unload HKU\DefaultUser2 2>&1 | Out-Null
 L 'Taskbar: search hidden, task view hidden, WT default terminal, no wallpaper'