error message 0VAR-COV OF SOME OBS ESTIMATED TO BE 0 - a bug?

Hello Nonmem Users, I am getting an error message: 0PROGRAM TERMINATED BY FNLETA ERROR IN CELS WITH INDIVIDUAL 2 ID= 1.10050000000000E+01 0VAR-COV OF SOME OBS ESTIMATED TO BE 0 MESSAGE ISSUED FROM TABLE STEP The estimate step is as follows: $EST MAXEVALS=9999 SIG=5 NOABORT PRINT=1 SORT METHOD=IMP CTYPE=3 NITER=1000 ISAMPLE=300 POSTHOC INTERACTION MSFO=KI4H004D.MSF LAPLACIAN The code also implements M3. The code runs fine, but it does not create the output data files specified in the table statements. I tried to remove some table statements, changed initial esimates, added the additive component to the error model. Nothing helps. When I do not use M3 and Laplacian, I am getting the output files. I used M3and Laplacian with some other drugs, but I did not use it in combination with METHOD=IMP. Can it be a NONMEM bug? The eror does not make sense. Thanks, Pavel