diff --git a/packaging/windows/build-installer.ps1 b/packaging/windows/build-installer.ps1
index 224275719..e5a98f526 100644
--- a/packaging/windows/build-installer.ps1
+++ b/packaging/windows/build-installer.ps1
@@ -55,10 +55,32 @@ if (-not $iscc) {
     $iscc = Find-Iscc
     if (-not $iscc) { throw 'winget finished but ISCC.exe was not found. Install Inno Setup 6 from https://jrsoftware.org/isdl.php and run again.' }
 }
-$isccVersion = (Get-Item $iscc).VersionInfo.FileVersion
-Say "Inno Setup $isccVersion at $iscc"
-$v = [Version]($isccVersion -replace '[^0-9.].*$', '')
-if ($v -lt [Version]'6.3.0') { throw "Inno Setup $isccVersion is too old; 6.3 or newer is needed (winget upgrade --id JRSoftware.InnoSetup)." }
+# The version: ISCC.exe's own file version first; some installs (the GitHub runner image) ship an ISCC.exe without a
+# version block (0.0.0.0), then the uninstall key's DisplayVersion or ISCmplr.dll next to it. Unknown = continue.
+function Get-IsccVersion([string]$exe) {
+    $candidates = @((Get-Item $exe).VersionInfo.FileVersion, (Get-Item $exe).VersionInfo.ProductVersion)
+    foreach ($key in @('HKLM:\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall\Inno Setup 6_is1',
+                       'HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\Inno Setup 6_is1',
+                       'HKCU:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\Inno Setup 6_is1')) {
+        try { $candidates += (Get-ItemProperty -Path $key -ErrorAction Stop).DisplayVersion } catch { }
+    }
+    $dll = Join-Path (Split-Path -Parent $exe) 'ISCmplr.dll'
+    if (Test-Path $dll) { $candidates += (Get-Item $dll).VersionInfo.FileVersion }
+    foreach ($c in $candidates) {
+        if (-not $c) { continue }
+        $clean = ($c -replace '[^0-9.].*$', '')
+        try { $v = [Version]$clean } catch { continue }
+        if ($v.Major -gt 0) { return $v }
+    }
+    return $null
+}
+$v = Get-IsccVersion $iscc
+if ($v) {
+    Say "Inno Setup $v at $iscc"
+    if ($v -lt [Version]'6.3.0') { throw "Inno Setup $v is too old; 6.3 or newer is needed (winget upgrade --id JRSoftware.InnoSetup)." }
+} else {
+    Say "Inno Setup at $iscc (version unknown: no version block, no uninstall key; continuing)"
+}
 
 # ---- 2. the art ------------------------------------------------------------------------------------------------------
 $art = $null