Fatal Error – ctype_digit()

I just upgraded WordPress to 2.5, and now I keep getting a fatal error. I get the following error message when uploading photos and in the Media Library section under “Location”.

Fatal error: Call to undefined function: ctype_digit() in /*****/wp-includes/link-template.php on line 182

The photos actually get uploaded to the server just fine, but rather than seeing the pretty uploading animations, I get this error. Also, it only seems to be showing the newest image in the media library list, probably because it hits this error and can’t continue displaying the rest of the page (the footer is missing as well).

I tried deleting all the files and re-uploading them, but it didn’t change anything.

I don’t have any plugins activated, and the only two installed are the default Akismet and Hello Dolly. I have also switched back to using the default theme and have no widgets activated.