I only use host headers............

I have created a top-level site  underneath a WSS site that is using host headers. However when I go to use the following code:

SPSite siteCollection = new SPSite(collection);

and the collection  is set to http://sites.server/sites/clients" target="_blank" rel="nofollow">http://sites.server/sites/clients I get the message saying "There is no Web named "/sites/clients". I used IE to navigate to http://sites.server/sites/clients" target="_blank" rel="nofollow">http://sites.server/sites/clients and the top-level web is there.

I created another collection under a different WSS site and I get no error  when the collection is http://server/sites/clients.

Any ideas?

Is this the exact message? There is no Web named "/sites/clients"

The reason that I ask is because you are trying to create  an SPSite
object when the error  occurs. That seems odd. How are you getting the
top-level site  from the siteColleciton object?

SPWeb site = siteCollection.RootWeb();

Or:
SPWeb site = siteCollection.OpenWeb("");

Or:
SPWeb site = siteCollection.AllWebs("");

This is the exact code that I have:

[WebMethod]
public string AddSite(string collection, string url, string title, string description, string template) {
try {
SPSite siteCollection = new SPSite(collection);
SPWeb site  = siteCollection.AllWebs[""];
SPWebCollection subSites = site.Webs;
SPWeb newWeb = subSites.Add(url, title, description, 1033, template, false, false);
return newWeb.Url.ToString();
} catch (Microsoft.SharePoint.SPException ee) {
ww.return ee.Message + "::" + ee.Source;
} catch (System.Exception ee) {
return ee.Message + "::" + ee.Source;
}
}

I should probably clarify the exception. No exception actually occurs when I execute the line SPSite siteCollection = new SPSite(collection). However, when I inspect the properties of the siteCollection object  here is an example of what I see in the QuickWatch window:

siteCollection.Owner = 'siteCollection.Owner' threw an exception of type 'Microsoft.SharePoint.SPException'
siteCollection.Owner.base.Message = There is no Web named "/sites/clients".

On another property:

siteCollection.AllWebs.Count = 'siteCollection.AllWebs.Count' threw an exception of type 'Microsoft.SharePoint.SPException'
siteCollection.AllWebs.Count.base.Message = "Operation aborted (Exception from HRESULT: 0x80004004 (E_ABORT))"

When I access another site that isn't using the host headers  I get those properties filled in with values.

What if you change AllWebs[""] to RootWeb()?

I tried to change it and I get the same problem. Would it not still fail because the line before that to obtain the siteCollection failed?