diff --git a/installer/Dragon-ISO.Installer.wixproj b/installer/Dragon-ISO.Installer.wixproj
index c406e93..e286bfe 100644
--- a/installer/Dragon-ISO.Installer.wixproj
+++ b/installer/Dragon-ISO.Installer.wixproj
@@ -17,8 +17,17 @@
     -->
     <PublishDir>$(MSBuildThisFileDirectory)..\publish\Dragon-ISO\</PublishDir>
 
+    <!--
+      MSI ProductVersion must be numeric x.y.z (no -beta/-rc suffix), and it is
+      what MajorUpgrade compares — so it must track the real build version, not
+      a hardcoded literal. Strip any prerelease suffix from $(Version). Note:
+      two prereleases of the same x.y.z share a ProductVersion and won't
+      upgrade over each other; bump the patch digit between betas.
+    -->
+    <MsiProductVersion>$([System.Text.RegularExpressions.Regex]::Match('$(Version)', '^\d+\.\d+(\.\d+)?').Value)</MsiProductVersion>
+
     <!-- Pass MSBuild values into WiX preprocessor. -->
-    <DefineConstants>PublishDir=$(PublishDir);AssetsDir=$(MSBuildThisFileDirectory)..\src\Dragon-ISO.App\Assets\</DefineConstants>
+    <DefineConstants>PublishDir=$(PublishDir);AssetsDir=$(MSBuildThisFileDirectory)..\src\Dragon-ISO.App\Assets\;ProductVersion=$(MsiProductVersion)</DefineConstants>
 
     <!-- Code-signing hook (set externally for release builds; left empty for dev). -->
     <SignOutput Condition=" '$(SignOutput)' == '' ">false</SignOutput>
diff --git a/installer/Package.wxs b/installer/Package.wxs
index b85e1a3..6a65a73 100644
--- a/installer/Package.wxs
+++ b/installer/Package.wxs
@@ -23,7 +23,7 @@
 
   <Package Name="Dragon-ISO"
            Manufacturer="Wild Dragon LLC"
-           Version="1.0.0.0"
+           Version="$(var.ProductVersion)"
            UpgradeCode="9F4A8B2C-1D3E-4A5B-9C6D-8E7F0A1B2C3D"
            Scope="perMachine"
            Compressed="yes"