diff --git a/repo2docker/contentproviders/doi.py b/repo2docker/contentproviders/doi.py
index 5769b2ff..ee51a83e 100644
--- a/repo2docker/contentproviders/doi.py
+++ b/repo2docker/contentproviders/doi.py
@@ -62,7 +62,7 @@ def doi2url(self, doi):
                 # default Git provider as this leads to a misleading error.
                 self.log.error(f"DOI {doi} does not resolve: {e}")
                 raise
-            return resp.headers['Location']
+            return resp.headers["Location"]
         else:
             # Just return what is actulally just a URL
             return doi