diff --git a/lib/unicorn/unicorn_json_log_formatter.rb b/lib/unicorn/unicorn_json_log_formatter.rb
index 4410c467bdc..7d68d7368e9 100644
--- a/lib/unicorn/unicorn_json_log_formatter.rb
+++ b/lib/unicorn/unicorn_json_log_formatter.rb
@@ -5,7 +5,7 @@ class UnicornJSONLogFormatter < Logger::Formatter
     default = {
       severity: severity,
       datetime: datetime,
-      progname: progname,
+      progname: progname || '',
       pid: $$,
     }