diff --git a/ConvertTo-ExcelXlsx.ps1 b/ConvertTo-ExcelXlsx.ps1
index 431924f..16cebb8 100644
--- a/ConvertTo-ExcelXlsx.ps1
+++ b/ConvertTo-ExcelXlsx.ps1
@@ -20,7 +20,7 @@ PARAM
     $xlsFile = Get-Item -Path $Path
     $xlsxPath = "{0}x" -f $xlsFile.FullName
 
-    if($xlsFile.Extension.ToLower() -ne ".xls"){
+    if($xlsFile.Extension -ne ".xls"){
         throw "Expected .xls extension"
     }