下面的程序无法排序，为什么？


PROC SORT DATA = aa OUT = Ab;
BY code  date;
RUN;


输出结果是：

540  PROC SORT DATA = aa OUT = Ab;
541
542  BY code  date;
543  RUN;
ERROR: Object creation failure.
NOTE: The SAS System stopped processing this step because of errors.
WARNING: The data set WORK.AB may be incomplete.  When this step was stopped there were 0
         observations and 15 variables.
WARNING: Data set WORK.AB was not replaced because this step was stopped.
NOTE: PROCEDURE SORT used (Total process time):
      real time           0.04 seconds
      cpu time            0.00 seconds