diff --git a/lib/synth-audio.js b/lib/synth-audio.js
index fda887f..c3a5e78 100644
--- a/lib/synth-audio.js
+++ b/lib/synth-audio.js
@@ -278,7 +278,7 @@ const synthPolly = async(logger, {credentials, stats, language, voice, engine, t
       });
     } else if (roleArn) {
       const stsClient = new STSClient({ region});
-      const roleToAssume = { RoleArn: roleArn, RoleSessionName: 'Jambonz Speech' };
+      const roleToAssume = { RoleArn: roleArn, RoleSessionName: 'Jambonz_Speech' };
       const command = new AssumeRoleCommand(roleToAssume);
 
       const response = await stsClient.send(command);