Seems like you're just reaching a "block" on that particular sound. This is a disfluency that happens regularly for a lot of people who stutter, including myself. Saying something before it to smooth it out is usually called a "bridge word" and lets you sort of ease into it with a sound you can easily make. From my studies in the CSD field I suggest you try to not get used to using bridging words too much, with me it's become regular and it's hard to stop. Try to ease into the sound you get stuck on rather that a more hard-attack on it, if that makes sense. If it becomes a concern of yours, see if you can receive therapy from a licensed speech-language pathologist somewhere.