Process single Thymeleaf fragment with Spring Boot

In my Spring Boot application, I need to process a single Thymeleaf Fragment to get its rendered HTML output.

I'm injecting

@Autowired
private SpringTemplateEngine templateEngine;

and then try to

Context context = new Context();
context.setVariable("key", value);

String html = this.templateEngine.process("fragments/foo :: bar(key=${key})", context);

with foo.html located in src/main/resources/templates/fragments:

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml"
    xmlns:th="http://www.thymeleaf.org"
    xmlns:layout="http://www.ultraq.net.nz/thymeleaf/layout">
<head></head>
<body>
...

<th:block th:fragment="bar(key)">
    <!-- doing fancy stuff with key... -->
</th:block>

...
</body>
</html>

I didn't change any autoconfigured values of Thymeleaf.

Running this code, I get the following exception:

org.thymeleaf.exceptions.TemplateInputException: Error resolving template "fragments/foo :: bar(key=${key})", template might not exist or might not be accessible by any of the configured Template Resolvers
    at org.thymeleaf.TemplateRepository.getTemplate(TemplateRepository.java:246) ~[thymeleaf-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1104) ~[thymeleaf-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1060) ~[thymeleaf-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1011) ~[thymeleaf-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:924) ~[thymeleaf-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:898) ~[thymeleaf-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    ...

Am I doing anything wrong here? The whole stuff gets working if I create a file src/main/resources/templates/fragments/test.html with simply the content

<div th:replace="fragments/foo :: bar(key=${key})"></div>

and process it via

String html = this.templateEngine.process("fragments/test", context);

That would be a possible solution, but seems a bit hackish to me...