After removing index.php from URLs, any segment will load homepage...help!

ExpressionEngine is loaded into the server root (ie. public_html, not a sub-folder).

Here’s the rewrite block from my .htaccess:

<IfModule mod_rewrite.c>
    RewriteEngine On

    # Removes index.php
    RewriteCond $1 !\.(gif|jpe?g|png)$ [NC]
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule ^(.*)$ /index.php/$1 [L]

    # If 404s, "No Input File" or every URL returns the same thing
    # make it /index.php?/$1 above (add the question mark)
</IfModule>

I have template groups for the various pages, so my URL’s look like: domain.com/<template_group>

However, if someone types in domain with a nonexistant template group (eg. domain.com/morecowbell), it will still load the home page—rather than firing a 404 or other server error. Can anyone help me troubleshoot this?

I noticed this because Google has indexed domain.com/blog somehow, which doesn’t exist on my site, it just loads the home page.