diff --git a/Library/Homebrew/api/formula.rb b/Library/Homebrew/api/formula.rb
index a70fe31b8c..02427b072c 100644
--- a/Library/Homebrew/api/formula.rb
+++ b/Library/Homebrew/api/formula.rb
@@ -33,7 +33,7 @@ module Homebrew
             if cached_formula_json_file.exist?
               last_modified = cached_formula_json_file.mtime.utc
               last_modified = last_modified.strftime("%a, %d %b %Y %H:%M:%S GMT")
-              curl_args = ["--header", "If-Modified-Since: #{last_modified}", *curl_args]
+              curl_args = ["--time-cond", last_modified, *curl_args] unless cached_formula_json_file.empty?
             end
             curl_download(*curl_args, to: HOMEBREW_CACHE_API/"#{formula_api_path}.json", max_time: 5)