Length cannot be less than zero. Parameter name: length" getting error in powershell

$command1 = 'git shortlog branch..branch1 --oneline --pretty="%an committed %h %s"'
$A=iex $command1


Write-Host "Starting"

Write-Host "testing "
foreach($i in $A)
{$pos = $i.IndexOf(":")
$leftPart = $i.Substring(0, $pos)
$rightPart = $i.Substring($pos+1)
$output += $rightPart
}
$output

Write-Host "SEPERATED"

$c=$rightPart|Select-Object -Unique
$c

Exception calling "Substring" with "2" argument(s): "Length cannot be less than zero.
Parameter name: length"
At line:10 char:1
+ $leftPart = $i.Substring(0, $pos)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : ArgumentOutOfRangeException

foreach($i in $A)