LDAP Query Returning Erroneous Values

I'm trying to query an LDAP server from a stored procedure written for the CLR but not getting the expected results.

The code is as follows:

<Microsoft.SqlServer.Server.SqlProcedure()> _
Public Shared Sub LDAP_UserExists(<Out()> ByRef exists As Boolean, ByVal username As SqlString)

Dim adspath As New StringBuilder()
adspath.Append(LDAP://[.......]/ou=Members/cn=)
adspath.Append(username)

If username.ToString().Length > 0 Then
Dim uobject As New DirectoryEntry(adspath.ToString(), "", "", System.DirectoryServices.AuthenticationTypes.Anonymous)
If Not (uobject Is Nothing) Then
exists = True
Else
exists = False
End If
End If

End Sub

The same code works fine from an ASP.NET. If I deploy the code and execute it with

exec LDAP_UserExists 'username'

I receive the error

Error converting data type varchar to bit.

And if I right-click and select "Execute Stored Procedure..." I receive @exists = 1 and Return Value = 0, regardless of the value I pass in as the username parameter.

Given that the same code works correctly on the ASP.NET page I suspect that this error has something to do with the <out()> parameter in the stored procedure declaration.

Can anyone suggest the correct method of performing this query?