diff --git a/repo2docker/buildpacks/repo2docker-entrypoint b/repo2docker/buildpacks/repo2docker-entrypoint
index f88215c3..e679173f 100755
--- a/repo2docker/buildpacks/repo2docker-entrypoint
+++ b/repo2docker/buildpacks/repo2docker-entrypoint
@@ -30,7 +30,7 @@ def main():
     if "REPO_DIR" in os.environ:
         log_dirs.insert(0, os.environ["REPO_DIR"])
     for d in log_dirs:
-        log_path = join(d, 'log.txt')
+        log_path = os.path.join(d, 'log.txt')
         try:
             log_file = open(log_path, "ab")
         except PermissionError: