You'll hit this assertion in WKStringCopyCFString: ASSERT(!toImpl(stringRef)->string().isNull()); on submitting a form at http://webkit.org/new-bug if you convert the values in the WKDictionaryRef passed in the willSubmitForm callback to CFStringRefs using WKStringCopyCFString due to some of the strings being null WTF::Strings.
<rdar://problem/8662180>
Created attachment 73757 [details] Patch
Fixed in r71930.
No test? :-(