Am I a racist? What do you think?

I am a 55-year old white male who lives in a quiet neighborhood in a quiet city in Maryland. I have lived in the same house for 24 years and never had trouble of any kind. There are blacks, whites, Asians, and one Egyptian family living here and I am good friends with all of them. However, in the past four months I have been held up, at gunpoint, twice, each time by two separate trios of black teen-aged boys at night while walking my dog within a block of my home. Now, I find myself very suspicious of all young African-American males and don't trust any of them at all. I have no trouble dealing with adults of all races but young black boys make me nervous. My friends (blacks included) tell me that there's nothing wrong with reacting the way I do but I don't like this feeling. Tell me what you think. Am I a racist for distrusting young black males?