StringConcatFactory.makeConcatWithConstants throws AssertionError when recipe contains non-String constants

The below code snippet when compiled and run on JDK9, throws Assertion error.

        StringConcatFactory.makeConcatWithConstants(lookup(), "name",
                MethodType.methodType(String.class, String.class, String.class), "\1\2\1", (Integer)1);

Though method is intended as BSM for indyfying string concatenation, javac seems to never produce a recipe with non-string constants. But when used directly as an API, recipe can be provided any constants as per javadoc.