Could not get SPWeb so could not get user key.

We were seeing the following errors in the ULS during a search crawl using a custom content enrichment web service (CEWS). Unexpected: "Could not get SPWeb so could not get user key." Medium: "No user key for user in request usage." Our CEWS was running in an anonymously accessible site in a specific web application for our search center. The search center web application was running on legacy Windows authentication. The content we were indexing was in a different web application that was running on Claims authentication. The solution to our problem was convert our search center web application to Claims as well. This can easily be done using PowerShell: Convert-SPWebApplication -Identity [web application] -From Legacy -To Claims