When a char[] passed to String.IndexOfAny in a LINQ Where filter is empty or null, StringExpressionToRegexFilterTranslator emits a malformed character class. anyOf.Length == 0 skips the single-character branch and falls into the else at StringExpressionToRegexFilterTranslator.cs:607, which emits [

] with an empty set. A null array throws from the foreach in EscapeCharacterSet.

x.Str.IndexOfAny(Array.Empty<char>()) == 0 rendered as ^[] and causes server error: "Regular expression is invalid: missing terminating ] for character class."

 x.Str.IndexOfAny(null) == 0 failed client-side with NullReferenceException.

 Expected results: IndexOfAny with an empty array always returns -1, so comparison should match no documents.