Problem in RewriteRule (HTACCESS)

I have the following HTACCESS code:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^customer/$ index.php [L,NC]
</IfModule>

(I cleaned the rest of the code for simplicity.)

In the folder HTACCESS is in (root), it has a customer.php

When I test, instead of "playing" for index.php , it "plays" for customer.php

If I change the code to, for example:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^customers/$ index.php [L,NC]
</IfModule>

or

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^qqercoisa/$ index.php [L,NC]
</IfModule>

It runs successfully.

Could anyone help me?