很愚蠢的一个问题

50.The following SAS program is submitted:
data test;
input country $8. date mmddyy10.;
cards;
Germany 12/31/2000
France 01/32/2001
;
run;
Which one of the following is the value of the variable _ERROR_ when the variable _N_
has a value of 2?
a. 0
b. 1
c. true
d. false



Correct answer: b
_N_ and _ERROR_ are automatic variables that can be used for DATA step processing
but that are not written to the output data set. _N_ counts the number of times that the
DATA step begins to execute, and _ERROR_ signals the occurrence of an error that is
caused by the data during execution. A value of 0 indicates no error while a value of 1
indicates one or more errors. In the program above, the value for date in the second
observation is invalid.
You can learn about the automatic variables _N_ and _ERROR_ in Understanding
DATA Step Processing.

这个为什么会invalid?我确实不明白