Query string argument of EXECUTE is null - postgresql

Question

EDIT
It seems my issue is when this select statement returns null (which is the case I'm trying to handle - when it returns null, I want my new value to be -999). How can I go about doing this if it errors out whenever a null is found?
ORIGINAL
I have read every other SO post I could find regarding this error, but none of which seemed to address the root of my issue.
The error is pretty straightforward - one of my arguments within my EXECUTE statement is null. Great. However, I print out each of the values that make up my EXECUTE statement right before it gets called, and I can clearly see that none of the values are null.
Code:
CREATE FUNCTION inform_icrm_prob_flow_query(tablename text, location_id int,
product_date_str text, lead_time_start int,
lead_time_end int, first_member_id int,
last_member_id int, dest_file text)
RETURNS void AS $$
DECLARE
count int;
product_date TIMESTAMPTZ;
interval_lead_time_start text;
interval_lead_time_end text;
curr_value double precision;
query text;
BEGIN
product_date := product_date_str::TIMESTAMPTZ;
count := first_member_id;
curr_value := 0;
interval_lead_time_start := ''''|| product_date ||'''::timestamptz +
interval '''||lead_time_start||' hours''';
interval_lead_time_end := ''''|| product_date ||'''::timestamptz +
interval '''||lead_time_end||' hours'' -
interval ''6 hours''';
--create our temporary table and populate it's date column
EXECUTE 'CREATE TEMPORARY TABLE temp_table_icrm_prob_flow AS
SELECT * FROM generate_series('||interval_lead_time_start || ',' ||
interval_lead_time_end || ', ''6 hours'')
AS date_valid';
LOOP
EXIT WHEN count > last_member_id;
IF NOT EXISTS(
SELECT 'date_valid'
FROM information_schema.columns
WHERE table_name='temp_table_icrm_prob_flow'
and column_name='value'||count||'')
THEN
EXECUTE 'ALTER TABLE temp_table_icrm_prob_flow ADD COLUMN value' || count
|| ' double precision DEFAULT -999';
END IF;
raise notice 'tablename: %', tablename;
raise notice 'location_id: %', location_id;
raise notice 'product_date: %', product_date;
raise notice 'count: %', count;
query := 'SELECT value FROM '|| tablename ||'
INNER JOIN temp_table_icrm_prob_flow
ON (temp_table_icrm_prob_flow.date_valid = '|| tablename ||'.date_valid)
WHERE '|| tablename ||'.id_location = '|| location_id ||'
AND '|| tablename ||'.date_product = '''|| product_date ||'''
AND '|| tablename ||'.id_member = '|| count ||'';
EXECUTE query INTO curr_value;
EXECUTE 'UPDATE temp_table_icrm_prob_flow
SET value'|| count ||' = COALESCE('|| curr_value ||', -999)';
count := count + 1;
END LOOP;
EXECUTE 'ALTER TABLE temp_table_icrm_prob_flow DROP COLUMN date_valid';
EXECUTE 'COPY temp_table_icrm_prob_flow TO '''||dest_file||''' DELIMITER '','' CSV';
EXECUTE 'DROP TABLE temp_table_icrm_prob_flow';
END;
$$ LANGUAGE plpgsql;
Output:
NOTICE: tablename: inform_tseries_data_basin_proc_fcst_prob_flow
NOTICE: location_id: 38
NOTICE: product_date: 2015-02-05 12:00:00+00
NOTICE: count: 1
ERROR: query string argument of EXECUTE is null
CONTEXT: PL/pgSQL function inform_icrm_prob_flow_query(text,integer,text,integer,integer,integer,integer,text) line 38 at EXECUTE
If none of the variables I am passing in are null, and the only other thing referenced is a temp table that I know exists, what could be causing this error?
Note: when changing my query to:
query := 'SELECT value FROM '|| tablename ||' WHERE '|| tablename ||'.id_location = '|| location_id ||' AND '|| tablename ||'.date_product = '''|| product_date ||''' AND '|| tablename ||'.id_member = '|| count ||' AND temp_table_icrm_prob_flow.date_va lid = '|| tablename ||'.date_valid';
I get the following error:
NOTICE: tablename: inform_tseries_data_basin_proc_fcst_prob_flow
NOTICE: location_id: 38
NOTICE: product_date: 2015-02-05 12:00:00+00
NOTICE: count: 1
ERROR: missing FROM-clause entry for table "temp_table_icrm_prob_flow"
LINE 1: ..._data_basin_proc_fcst_prob_flow.id_member = 1 AND temp_table...
^
QUERY: SELECT value FROM inform_tseries_data_basin_proc_fcst_prob_flow WHERE inform_tseries_data_basin_proc_fcst_prob_flow.id_location = 38 AND inform_tseries_data_basin_proc_fcst_prob_flow.date_product = '2015-02-05 12:00:00+00' AND inform_tseries_data_basin_proc_fcst_prob_flow.id_member = 1 AND temp_table_icrm_prob_flow.date_valid = inform_tseries_data_basin_proc_fcst_prob_flow.date_valid
CONTEXT: PL/pgSQL function inform_icrm_prob_flow_query(text,integer,text,integer,integer,integer,integer,text) line 35 at EXECUTE

Answer 1

Sorry for small offtopic. Your code is pretty unreadable (and SQL injecttion vulnerable). There are some techniques, that you can use:
Use clause USING of EXECUTE statement for usual parameters.
DO $$
DECLARE
tablename text := 'mytab';
from_date date := CURRENT_DATE;
BEGIN
EXECUTE 'INSERT INTO ' || quote_ident(tablename) || ' VALUES($1)'
USING from_date;
END
$$;
This code will be safe (due using quote_ident function), little bit faster (due using binary value of from_date variable - removed multiple string<->date conversions and little bit more readable (because string expression is shorter).
Use function format. The building query string will be shorter and more readable (table aliases helps too):
query := format('
SELECT value
FROM %I _dtn
INNER JOIN temp_table_icrm_prob_flow t ON t.date_valid = _dtn.date_valid
WHERE _dtn.id_location = $1
AND _dtn.date_product = $2
AND _dtd.id_member = $3'
, tablename);
EXECUTE query INTO curr_value USING location_id, product_date, count;
Using variables named like important SQL keywords and identifier is wrong idea - names count, values are wrong.
The error message is clean - you are using the identifier temp_table_icrm_prob_flow.date_valid, but the table temp_table_icrm_prob_flow is not mentioned in query. The query missing JOIN part.