diff --git a/README.md b/README.md
index 4430cc03..dea330db 100644
--- a/README.md
+++ b/README.md
@@ -165,7 +165,8 @@ mkcert filippo@example.com
 
 ### Mobile devices
 
-For the certificates to be trusted on mobile devices, you will have to install the root CA. It's the `rootCA.pem` file in the folder printed by `mkcert -CAROOT`.
+For the certificates to be trusted on mobile devices, you will have to install the root CA. It's the `rootCA.pem` file in the folder printed by `mkcert -CAROOT`. However, you have to copy `rootCA.pem` to `rootCA.crt` since iOS does not read the `.pem` extension.
+
 
 On iOS, you can either use AirDrop, email the CA to yourself, or serve it from an HTTP server. After opening it, you need to [install the profile in Settings > Profile Downloaded](https://github.com/FiloSottile/mkcert/issues/233#issuecomment-690110809) and then [enable full trust in it](https://support.apple.com/en-nz/HT204477).