diff --git a/Rakefile b/Rakefile
index 7d5dbc0..aa4c70f 100644
--- a/Rakefile
+++ b/Rakefile
@@ -134,7 +134,10 @@ namespace :docker do
       return File.read(cache_path)
     end
 
-    head_hash = `curl -H 'accept: application/vnd.github.v3.sha' https://api.github.com/repos/ruby/ruby/commits/master`.chomp
+    head_hash = `curl -fs -H 'accept: application/vnd.github.v3.sha' https://api.github.com/repos/ruby/ruby/commits/master`.chomp
+    unless $?.success?
+      raise "get_ruby_master_head_hash failed: #{head_hash.inspect}"
+    end
     if cache_path
       File.write(cache_path, head_hash)
     end