--- a/python/mozbuild/mozbuild/frontend/reader.py
+++ b/python/mozbuild/mozbuild/frontend/reader.py
@@ -1230,34 +1230,33 @@ class BuildReader(object):
         Returns a dict of input paths to a list of relevant moz.build files.
         The root moz.build file is first and the leaf-most moz.build is last.
         """
         root = self.config.topsrcdir
         result = {}
 
         @memoize
         def exists(path):
-            return self._finder.get(path) is not None
+            return self._relevant_mozbuild_finder.get(path) is not None
 
         def itermozbuild(path):
             subpath = ''
             yield 'moz.build'
             for part in mozpath.split(path):
                 subpath = mozpath.join(subpath, part)
                 yield mozpath.join(subpath, 'moz.build')
 
         for path in sorted(paths):
             path = mozpath.normpath(path)
             if os.path.isabs(path):
                 if not mozpath.basedir(path, [root]):
                     raise Exception('Path outside topsrcdir: %s' % path)
                 path = mozpath.relpath(path, root)
 
-            result[path] = [p for p in itermozbuild(path)
-                              if exists(mozpath.join(root, p))]
+            result[path] = [p for p in itermozbuild(path) if exists(p)]
 
         return result
 
     def read_relevant_mozbuilds(self, paths):
         """Read and process moz.build files relevant for a set of paths.
 
         For an iterable of relative-to-root filesystem paths ``paths``,
         find all moz.build files that may apply to them based on filesystem