How to call powershell function properly?

I wrote a function to convert the input month parameter to a certain format. such as if I passed 04 to the funtion and the function will return the "_APR_". the function I wrote is like below:

function GetEnMonth()
 {
 param([string] $month)
 switch ($month)
 {
     ($_ -eq "01"){$result = "_JAN_"}
     ($_ -eq "02"){$result = "_FEB_"}
     ($_ -eq "03"){$result = "_MAR_"}
     ($_ -eq "04"){$result = "_APR_"}
     ($_ -eq "05"){$result = "_MAY_"}
     ($_ -eq "06"){$result = "_JUN_"}
     ($_ -eq "07"){$result = "_JUL_"}
     ($_ -eq "08"){$result = "_AUG_"}
     ($_ -eq "09"){$result = "_SEP_"}
     ($_ -eq "10"){$result = "_OCT_"}
     ($_ -eq "11"){$result = "_NOV_"}
     ($_ -eq "12"){$result = "_DEC_"}
     default {$result ="_No_Result_"}
 }
 return [string]$result;
 } 
 

Then I use below command to excute the function to get the result:

$mYear = $today.substring(0,4) 
 $mMonth =$today.substring(4,2)
 $mDate = $today.substring(6,2)
 $monthInEn = GetEnMonth $mMonth
 

well, the result is always "_No_Result_", why? below is the exception:

 **** Exception type : System.Management.Automation.RuntimeException
  **** Exception message : You cannot call a method on a null-valued expression.
 

Could anyone give me an answer for this? thx.
I have searched goole a lot but don't find useful solutions.

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