The current version of svn does not seem to indicate an error properly when passed an invalid string, so it's important to check your paths and to clean them before calling svn or it will just not work.
Under Windows getting the format correctly is a bit tricky, so here's the format it wants:
Note the following:
1) The extra slash after the protocol designates 'root' under Unix (file:///path/to/repos points to /path/to/repos in Unix), but it seems to have no real meaning in the string on Windows (?).
2) Paths are usually reported with backslashes, so you'll need to replace those with slashes manually before handing them off to Subversion.