Help with error: Unexpected token in expression or statement

Hi,

For days the following line in my code worked, with all variables containing valid data.  Im just concatenting variables:

$strADMTIncludeFile1 = $strADMTIncludeFile$strDateTime$strTxtFileXtension

Now I get the following error.  Any ideas as to why?

Unexpected token 'strDateTime' in expression or statement.
At C:\MigrateSIDHistory.ps1:74 char:55
+ $strADMTIncludeFile1 = $strADMTIncludeFile$strDateTime <<<< $strTxtFileXtension
    + CategoryInfo          : ParserError: (strDateTime:String) [], ParentContainsErrorRecordException
    + FullyQualifiedErrorId : UnexpectedToken

Thanks in advance.