Please post a testable page for us. For example, the following page works fine for me.
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
<c:catch var="error">
<fmt:parseNumber var="parsedNum" value="aaa" />
</c:catch>
${parsedNum}
${error}