CORS fail/working on alternating requests

gistfile1.txt

[info] [<0.2221.0>] 127.0.0.1 - - GET /dashboard/_design/dashboard/_view/dashboard_assets?reduce=false&include_docs=true 304
[error] [<0.2221.0>] function_clause error in HTTP request
[info] [<0.2221.0>] Stacktrace: [{string,to_lower,
                                  [undefined],
                                  [{file,"string.erl"},{line,468}]},
                                 {couch_httpd_cors,maybe_apply_cors_headers,
                                  2,
                                  [{file,
                                    "/Users/jan/Work/build-couchdb-mac/build-couchdb/git-build/https%3A%2F%2Fgit-wip-us.apache.org%2Frepos%2Fasf%2Fcouchdb.git%3A06c3f351f158f107a8293c7c6c9b2415b942f0e9/src/couchdb/couch_httpd_cors.erl"},
                                   {line,219}]},
                                 {couch_httpd,send_response,4,
                                  [{file,
                                    "/Users/jan/Work/build-couchdb-mac/build-couchdb/git-build/https%3A%2F%2Fgit-wip-us.apache.org%2Frepos%2Fasf%2Fcouchdb.git%3A06c3f351f158f107a8293c7c6c9b2415b942f0e9/src/couchdb/couch_httpd.erl"},
                                   {line,706}]},
                                 {couch_mrview_http,design_doc_view,5,
                                  [{file,
                                    "/Users/jan/Work/build-couchdb-mac/build-couchdb/git-build/https%3A%2F%2Fgit-wip-us.apache.org%2Frepos%2Fasf%2Fcouchdb.git%3A06c3f351f158f107a8293c7c6c9b2415b942f0e9/src/couch_mrview/src/couch_mrview_http.erl"},
                                   {line,147}]},
                                 {couch_httpd_db,do_db_req,2,
                                  [{file,
                                    "/Users/jan/Work/build-couchdb-mac/build-couchdb/git-build/https%3A%2F%2Fgit-wip-us.apache.org%2Frepos%2Fasf%2Fcouchdb.git%3A06c3f351f158f107a8293c7c6c9b2415b942f0e9/src/couchdb/couch_httpd_db.erl"},
                                   {line,234}]},
                                 {couch_httpd,handle_request_int,5,
                                  [{file,
                                    "/Users/jan/Work/build-couchdb-mac/build-couchdb/git-build/https%3A%2F%2Fgit-wip-us.apache.org%2Frepos%2Fasf%2Fcouchdb.git%3A06c3f351f158f107a8293c7c6c9b2415b942f0e9/src/couchdb/couch_httpd.erl"},
                                   {line,331}]},
                                 {mochiweb_http,headers,5,
                                  [{file,
                                    "/Users/jan/Work/build-couchdb-mac/build-couchdb/git-build/https%3A%2F%2Fgit-wip-us.apache.org%2Frepos%2Fasf%2Fcouchdb.git%3A06c3f351f158f107a8293c7c6c9b2415b942f0e9/src/mochiweb/mochiweb_http.erl"},
                                   {line,136}]},
                                 {proc_lib,init_p_do_apply,3,
                                  [{file,"proc_lib.erl"},{line,227}]}]
[info] [<0.2221.0>] 127.0.0.1 - - GET /dashboard/_design/dashboard/_view/dashboard_assets?reduce=false&include_docs=true 500
[error] [<0.2221.0>] httpd 500 error response:
 {"error":"unknown_error","reason":"function_clause"}

[info] [<0.2221.0>] 127.0.0.1 - - GET /dashboard/_design/dashboard/_view/dashboard_assets?reduce=false&include_docs=true 200