Using a SMO object as parameter value

It looks like a SMO object is implicit converted to a String object, when it is used in a function call by a parameter name.

I have a function, where the parameter is defined as
function Import-FileGroup {
param ([Microsoft.SqlServer.Management.Smo.Database]$smoDatabase = $(throw 'Value requered for parameter "smoDatabase" in function Import-FileGroup.'))

...
}

When I call the function with a parameter name
Import-FileGroup -$smoDatabase $smoDb
it fail with this message
Import-FileGroup : Cannot process argument transformation on parameter 'smoDatabase'. Cannot convert the "-" value of type "System.String" to type "Microsoft.SqlServer.Management.Smo.Database".
At U:\sqladmin\dn_sqladmin\Development\Import-Database.ps1:***** char:17
+ Import-FileGroup <<<<  -$smoDatabase $smoDb
    + CategoryInfo          : InvalidData: (:) [Import-FileGroup], ParameterBindin...mationException
    + FullyQualifiedErrorId : ParameterArgumentTransformationError,Import-FileGroup

But when I call the function without a parameter name
Import-FileGroup $smoDb
everything is working fine.

Not what I expected, but I have now made the note for future use...

The variable „$smoDb“ is created like this
$smoSrv = New-Object Microsoft.SqlServer.Management.Smo.Server 'SANDY.sqladmin.lan'
$smoDb = $smoSrv.Databases['my_database']