diff --git a/QuickLook.Installer/Product.wxs b/QuickLook.Installer/Product.wxs
index 306cb9a7..001c2b0c 100644
--- a/QuickLook.Installer/Product.wxs
+++ b/QuickLook.Installer/Product.wxs
@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
+<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi"
+     xmlns:util="http://schemas.microsoft.com/wix/UtilExtension">
   <Product Id="*" Name="QuickLook" Language="1033"
            Version="!(bind.FileVersion.fil22F7B7F07E63622AD2451459086A3CF2)" Manufacturer="QL-Win"
            UpgradeCode="c83b9c02-87d6-494e-9f5f-cf4c3900a54d">
@@ -60,6 +61,14 @@
       </Directory>
     </Directory>
 
+    <!-- Close QuickLook before install/upgrade/uninstall to prevent locked-file warnings -->
+    <util:CloseApplication Id="CloseQuickLookExe"
+                           Target="QuickLook.exe"
+                           CloseMessage="yes"
+                           RebootPrompt="no"
+                           TerminateProcess="1"
+                           Sequence="800" />
+
     <CustomAction Id="LaunchApplication"
                   FileKey="fil22F7B7F07E63622AD2451459086A3CF2"
                   ExeCommand="/first"
diff --git a/QuickLook.Installer/QuickLook.Installer.wixproj b/QuickLook.Installer/QuickLook.Installer.wixproj
index f704153a..dfcd0ad8 100644
--- a/QuickLook.Installer/QuickLook.Installer.wixproj
+++ b/QuickLook.Installer/QuickLook.Installer.wixproj
@@ -28,6 +28,10 @@
       <HintPath>$(WixExtDir)\WixUIExtension.dll</HintPath>
       <Name>WixUIExtension</Name>
     </WixExtension>
+    <WixExtension Include="WixUtilExtension">
+      <HintPath>$(WixExtDir)\WixUtilExtension.dll</HintPath>
+      <Name>WixUtilExtension</Name>
+    </WixExtension>
   </ItemGroup>
   <ItemGroup>
     <Content Include="gpl.rtf" />