diff --git a/Tools.ps1 b/Tools.ps1
index f5e160a..a1d2e29 100644
--- a/Tools.ps1
+++ b/Tools.ps1
@@ -798,7 +798,9 @@ Function Get-AbsoluteURI {
 Function Get-GUIDFromMSIUninstallString ([string]$str) {
     $start = $str.IndexOf('{') + 1
     $end = $str.IndexOf('}')
-    Return $str.Substring($start, $end - $start)
+    $retval = $str.Substring($start, $end - $start)
+    If ( $retval.Length -ne 36 ) { $retval = $null }
+    Return $retval
 }
 
 Function Uninstall-MSI ([Parameter(ValueFromPipeline=$true)][string[]]$APP_NAMES) {