Not Seeing Syntax Error

   For several INSERT INTO rows I get a syntax error when the quant column is
NULL for one specific parameter. I don't see my error. Here is an example
row:

psql:insert.sql:8: ERROR:  syntax error at or near ","
LINE 1: ...ALUES ('9609-0759','BC-1.5','1996-09-19','Arsenic',,'mg/L');
                                                               ^
   The source line is:

INSERT INTO chemistry (lab_nbr, loc_name, sample_date, param, quant,
units)VALUES ('9609-0759','BC-1.5','1996-09-19','Arsenic',,'mg/L');

and the quant column is defined as type real. There are numerous other rows
where quant IS NULL.

   What have I missed?

Rich