Saxon

declare context item := 'x';
declare variable $a := .;
1[$a]

… ends up in a Java exception:

java.lang.ClassCastException: class net.sf.saxon.value.StringValue cannot be cast to class net.sf.saxon.value.NumericValue (net.sf.saxon.value.StringValue and net.sf.saxon.value.NumericValue are in unnamed module of loader 'app')
        at net.sf.saxon.expr.SubscriptExpression.evaluateItem(SubscriptExpression.java:151)
        at net.sf.saxon.expr.Expression.process(Expression.java:940)
        at com.saxonica.ee.bytecode.ByteCodeCandidate.process(ByteCodeCandidate.java:146)
        ...