OGNL: An illegal reflective access operation has occurred

Running Struts on Java 11 gives a new warning about illegal reflection, not present on Java 8:

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by ognl.OgnlRuntime (file:/.../workspace/.../.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/.../WEB-INF/lib/ognl-3.1.21.jar) to method java.util.HashMap$Node.getKey()
WARNING: Please consider reporting this to the maintainers of ognl.OgnlRuntime
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

The warning appears after using the following tag in a JSP (name of properties changed):

<s:select name="item" key="item.key" list="items" />

The corresponding Action contains:

private Map<Long, String> items = new LinkedHashMap<>();