NONMEM 2 Compartment Zero Order Absorption

Hello all, I am currently trying to model data for a compound that is orally administered. A two compartment model with FIRST order oral absorption is not describing the data well (in my opinion). Consequently, I am attempting to model the data using the same two compartment model except with ZERO order absorption. I am under the impression that I am able to use ADVAN 4 TRANS 4 as well as a D1 = theta (1). I have included a rate = - 2 for my dosing records in my data set. My control stream is written as such: $INPUT ID DOSE DAY TIME=TAD NTPD DV AMT COH ADDL II MDV RATE EVID CMT LDV $SUBROUTINES ADVAN4 TRANS4 $PK TVD1 = THETA(1) D1 = TVD1*EXP(ETA(1)) TVCL=THETA(2) CL=TVCL*EXP(ETA(2)) TVV2=THETA(3) V2=TVV2*EXP(ETA(3)) TVQ = THETA(4) Q=TVQ*EXP(ETA(4)) TVV3=THETA(5) V3=TVV3*EXP(ETA(5)) However, I get the following error: WARNINGS AND ERRORS (IF ANY) FOR PROBLEM 1 (WARNING 2) NM-TRAN INFERS THAT THE DATA ARE POPULATION. AN ERROR WAS FOUND IN THE CONTROL STATEMENTS. THE CHARACTERS IN ERROR ARE: KA 196 $PK: NO VALUE ASSIGNED TO A BASIC PK PARAMETER. NMtran failed. There is no output for model 1. Contents of FMSG: WARNINGS AND ERRORS (IF ANY) FOR PROBLEM 1 (WARNING 2) NM-TRAN INFERS THAT THE DATA ARE POPULATION. AN ERROR WAS FOUND IN THE CONTROL STATEMENTS. THE CHARACTERS IN ERROR ARE: KA 196 $PK: NO VALUE ASSIGNED TO A BASIC PK PARAMETER. My question is whether I can utilize ADVAN 4 TRANS 4 to model zero order absorption for an oral administered drug (if so, how do I address the error listed above) OR if I have to utilize ADVAN 6? Thanks in advance for taking time out of your busy day to help! Regards, Bishoy Hanna Scientist Celgene Corporation 179 Passaic Avenue, S11- 2645A, Summit, NJ, 07901 Phone: (908) 897 - 7733 ********************************************************* THIS ELECTRONIC MAIL MESSAGE AND ANY ATTACHMENT IS CONFIDENTIAL AND MAY CONTAIN LEGALLY PRIVILEGED INFORMATION INTENDED ONLY FOR THE USE OF THE INDIVIDUAL OR INDIVIDUALS NAMED ABOVE. If the reader is not the intended recipient, or the employee or agent responsible to deliver it to the intended recipient, you are hereby notified that any dissemination, distribution or copying of this communication is strictly prohibited. If you have received this communication in error, please reply to the sender to notify us of the error and delete the original message. Thank You.