使用invoke sqlcmd在PowerShell中删除数据库会告诉我数据库正在使用中,即使它不是

我正试图删除这样的数据库

Invoke-Sqlcmd -Database Dbname-ServerInstance ".\SQLEXPRESS" -Query "drop database DbName;"

在PowerShell脚本中,我得到以下消息:

Invoke-Sqlcmd : Cannot drop database "DbName" because it is currently in use.
At line:1 char:1
+ Invoke-Sqlcmd -Database Dbname-ServerInstance ".\SQLEXPRESS" -Que ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo          : InvalidOperation: (:) [Invoke-Sqlcmd], SqlPowerShellSqlExecutionException
+ FullyQualifiedErrorId : SqlError,Microsoft.SqlServer.Management.PowerShell.GetScriptCommand

数据库未在使用中。我做错什么了?