Hi,
You may want to describe what "fun" is and the lower and upper bounds too. In that way, you may get better responses. However, I would like to refer you to this link. You can check the last answer here which can be of help. In addition, you may also try to set stopping criteria of "fminbnd" to suit your needs (see link). Also, refer to the documentation of the respective functions that you use to implement them properly. In case you need to understand what operations produce NaN, see this.
