diff --git a/distutils/command/sdist.py b/distutils/command/sdist.py
index e8abb73920..eda6afe811 100644
--- a/distutils/command/sdist.py
+++ b/distutils/command/sdist.py
@@ -391,7 +391,7 @@ def prune_file_list(self):
         build = self.get_finalized_command('build')
         base_dir = self.distribution.get_fullname()
 
-        self.filelist.exclude_pattern(None, prefix=build.build_base)
+        self.filelist.exclude_pattern(None, prefix=os.fspath(build.build_base))
         self.filelist.exclude_pattern(None, prefix=base_dir)
 
         if sys.platform == 'win32':