I am running into a failure with the LDAP Input step and compound Filter Strings.

We are connecting to Windows Active Directory via LDAP (which is working pretty well on the PUC) and the following Filter String works just fine:
(&(objectClass=User)(objectClass=computer))

However I need to return items that are not computers, so I tried:
(&(objectClass=User)(!objectClass=computer))
and got the following error:
LDAP Input.0 - ERROR (version 4.1.2-GA, build 14760 from 2011-01-26 13.31.23 by buildguy) : Unexpected error :
LDAP Input.0 - ERROR (version 4.1.2-GA, build 14760 from 2011-01-26 13.31.23 by buildguy) : org.pentaho.di.core.exception.KettleException:
LDAP Input.0 - ERROR (version 4.1.2-GA, build 14760 from 2011-01-26 13.31.23 by buildguy) : !LDAPConnection.Error.Search!
LDAP Input.0 - ERROR (version 4.1.2-GA, build 14760 from 2011-01-26 13.31.23 by buildguy) : Unbalanced parenthesis
LDAP Input.0 - ERROR (version 4.1.2-GA, build 14760 from 2011-01-26 13.31.23 by buildguy) : org.pentaho.di.trans.steps.ldapinput.LDAPConnection.search(LDAPConnection.java:280)
LDAP Input.0 - ERROR (version 4.1.2-GA, build 14760 from 2011-01-26 13.31.23 by buildguy) : org.pentaho.di.trans.steps.ldapinput.LDAPInput.search(LDAPInput.java:382)
LDAP Input.0 - ERROR (version 4.1.2-GA, build 14760 from 2011-01-26 13.31.23 by buildguy) : org.pentaho.di.trans.steps.ldapinput.LDAPInput.processRow(LDAPInput.java:80)
LDAP Input.0 - ERROR (version 4.1.2-GA, build 14760 from 2011-01-26 13.31.23 by buildguy) : org.pentaho.di.trans.step.RunThread.run(RunThread.java:40)
LDAP Input.0 - ERROR (version 4.1.2-GA, build 14760 from 2011-01-26 13.31.23 by buildguy) : java.lang.Thread.run(Thread.java:662)

I notice that the documentation says: " in the current stage only "single filter format" is supported"
http://wiki.pentaho.com/display/EAI/LDAP+Input
And while this comment might be related, the first Filter worked, so why not the second?

What gives?