HMACSHA1 Generating Different Hash for Same Input

Any reason why the following code might generate a different hash from the same input?
var baseStringBytes = Encoding.UTF8.GetBytes(baseString); var hash = HMACSHA1.Create().ComputeHash(baseStringBytes); Stepping through with the debugger shows that the baseString is the same each time, however, when I check the value of 'hash', the bytes are different each time.