Switch does not remove svn:externals

I have made a simple project showing an issue with the svn:externals property not being removed correctly from a folder when the folder is switched (see repo1.zip in Attachments)

Scenario:
My project contains:
branches
trunk

Within branches, there is a folder "b1" with a property "svn:externals" pointing to "file:///C:/temp/tes​t1/repo1/trunk/f1"

To reproduce:
- Put the repo in C:\temp\test1\repo1
- Checkout file:///C:/temp/test​1/repo1/trunk/ to C:\temp\test1\co. This creates the folder C:\temp\test1\co without properties
- Switch C:\temp\test1\co to
file:///C:/temp/test​1/repo1/branches/b1.​ This removes folder f1 and adds the external folder f1.
- Now try to switch back to trunk. This fails, as it probably should because we already have a folder f1 in the working copy.
- Remove folder f1 and try again to switch to trunk. This works...

...BUT the svn:externals property of C:\temp\test1\co is still set. This seems wrong. To me, it would be correct if the switch command would have removed the svn:externals property.

(Using TortoiseSVN 1.5.7, Build 15182 - 32 Bit , 2009/01/24 22:44:13, Subversion 1.5.5)