在用truncreg命令时，出现了如下的提示，不知如何解决，故请教高人！谢谢！

truncreg  exp income x ,ll(0)
(note: 4579 obs. truncated)
Fitting full model:
Iteration 0:   log likelihood = -39284.388  
Iteration 1:   log likelihood =   -37945.5  
Iteration 2:   log likelihood =  -36643.02  
Iteration 3:   log likelihood = -36434.545  
Iteration 4:   log likelihood = -36163.435  
Iteration 5:   log likelihood = -36052.784  
Iteration 6:   log likelihood = -36048.709  
Iteration 7:   log likelihood = -36047.034  
Iteration 8:   log likelihood = -36046.936  (backed up)
Iteration 9:   log likelihood = -36046.929  (backed up)
Iteration 10:  log likelihood = -36046.926  (backed up)
Iteration 11:  log likelihood = -36046.925  (backed up)
numerical derivatives are approximate
nearby values are missing
numerical derivatives are approximate
nearby values are missing
cannot compute an improvement -- discontinuous region encountered
r(430);