Arduino C++ programm locks up after sending "on" but not "off"

Question

I have a piece of code that is sending an MQTT message.

The message is the same with the only change being changing action between 'on' and 'off'. I can end off as many times as I like, but when I send 'on' it locks (even if it is the first message)

Here is the code

void update_thing(int pin, String thing, String action) {  

  Serial.println(thing + " State Requested to " + action);
  Serial.println(action.c_str());
    if (action == "On") {
      digitalWrite(pin, HIGH);                    // Pin  = 5 V, LED Turns On
    } else {
      digitalWrite(pin, LOW);                     // Pin  = 5 V, LED Turns Off     
    }

  // Generate a new message 
  sprintf(msg, "{\"state\":{\"reported\":{\"%s\": \"%s\"}}}", thing.c_str(), action.c_str());
  if((rc = myClient.publish("$aws/things/myYunLight/shadow/update", msg, strlen(msg), 1, false)) != 0) {
    Serial.println("Publish failed!");
    Serial.println(rc);
  }
}

The issue appears to lie in the msg variable, declared as

char msg[100]; // read-write buffer

and in particular the value fed to action (or even if I remove that and use the text 'on' or 'off'. If I use 'on ' (with a space) it works fine