The high court has given investors one more reason to ignore a fund’s documents, ruling that a fund’s investment adviser may not be sued for securities fraud due to misstatements made in a fund’s prospectus.

In a 5-4 decision last week, the high court tossed a lawsuit against Janus Capital Group Inc. JNS +0.44% , the sponsor of Denver-based Janus mutual funds. The court ruled that Janus Capital and a subsidiary that advises the funds could not be sued for supposedly misleading statements in the prospectuses of the Janus funds.

The court reasoned that because the parent company is a separate entity from the funds themselves, only the fund can be held to the Securities & Exchange Commission’s standard that “any person, directly or indirectly … [making] any untrue statement of material fact” in the buying or selling process of a security is breaking the law.