CodeIgniter base_url（）返回URL中的Windows路径

CodeIgniter 3.0.3 on IIS7 intranet server: I'm migrating an application from 2.x and following the upgrade procedure.

$config['base_url'] = 'http://myIntranet.myDomain.com/myAppDir/';

If I display $base_url() on my webpage I get

base_url: http://myIntranet.myDomain.com/myAppDir/C:\inetpub\wwwroot\myAppDir\application\ 

i.e., It's giving me C:\inetpub\wwwroot\myAppDir\application\ as part of the URL.

Access control (web.config) is as follows:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <rewrite>
            <rules>
                <rule name="Rewrite to index.php">
                    <match url="index.php|robots.txt|images|test.php" />
                    <action type="None" />
                </rule>
                <rule name="Rewrite CI Index">
                    <match url=".*" />
                    <conditions>
                        <add input="{REQUEST_FILENAME}" matchType="Pattern" pattern="css|js|jpg|jpeg|png|gif|ico|htm|html" ignoreCase="true" negate="true" />
                    </conditions>
                    <action type="Rewrite" url="index.php/{R:0}" />
                </rule>
            </rules>
        </rewrite>
        <httpErrors errorMode="Detailed" />
    </system.webServer>
</configuration>

I've read through multiple posts with similar problems on this site but haven't found exactly the same problem.

Can anyone point me in the right direction to fix?