diff --git a/.github/workflows/nightly.yml b/.github/workflows/nightly.yml
index 9acc9401e..d66377b1a 100644
--- a/.github/workflows/nightly.yml
+++ b/.github/workflows/nightly.yml
@@ -240,3 +240,19 @@ jobs:
             Write-Host "::error::install.ps1 installed a 0x$machine binary on this runner, expected 0x$env:PE_MACHINE"
             exit 1
           }
+
+      # The documented one-liner, which binds params differently from the
+      # scriptblock form above. It takes no flags, so it installs the
+      # latest stable release to the default location.
+      - name: install.ps1 via irm | iex (Windows PowerShell 5.1)
+        if: runner.os == 'Windows'
+        shell: powershell
+        run: |
+          irm "$env:RAW_BASE/install.ps1" | iex
+          $exe = Join-Path $env:USERPROFILE '.local\bin\q2.exe'
+          $raw = & $exe --version
+          Write-Host $raw
+          if ($LASTEXITCODE -ne 0 -or -not $raw) {
+            Write-Host "::error::q2.exe from the irm | iex install did not run"
+            exit 1
+          }
diff --git a/install.ps1 b/install.ps1
index 47c788b58..d3cfb5bc6 100644
--- a/install.ps1
+++ b/install.ps1
@@ -42,7 +42,7 @@ param(
     [string]$Checksum,
     [switch]$NoVerify,
     [switch]$Nightly,
-    [ValidateSet('windows_amd64', 'windows_arm64')]
+    # No [ValidateSet]: under `irm | iex` it rejects the empty default.
     [string]$Platform
 )
 
@@ -56,6 +56,10 @@ $UA = @{ 'User-Agent' = 'q2-install' }
 function Die([string]$msg) { throw "q2 install: $msg" }
 function Step([string]$msg) { Write-Host "-> $msg" }
 
+if ($Platform -and $Platform -notin @('windows_amd64', 'windows_arm64')) {
+    Die "unknown -Platform '$Platform' (expected windows_amd64 or windows_arm64)"
+}
+
 if (-not [System.Environment]::Is64BitOperatingSystem) {
     Die 'only 64-bit Windows is supported'
 }