PS I also tried start = list( fixef = c( -9.28405, 2.81300, -4.75935, 2.91080), ST = c(0, 8.29931, 2.56368e-06, 0.00000)) and start = list( fixef = list( -9.28405, 2.81300, -4.75935, 2.91080), ST = list(0, 8.29931, 2.56368e-06, 0.00000)) to no avail.