PostgreSQL error: query string argument of EXECUTE is null

I have a table called evidence with a trigger which calls a stored procedure which basically does table partitioning by month. However I get an obscure error when I start inserting lots of rows under load:

Npgsql.NpgsqlException: query string argument of EXECUTE is null 
Severity: ERROR Code: 22004 at Npgsql.NpgsqlState.<ProcessBackendResponses_Ver_3>d__a.MoveNext() in c:\C#Apps\github.npgsql.Npgsql.stock\src\Npgsql\NpgsqlState.cs:line890 at Npgsql.ForwardsOnlyDataReader.GetNextResponseObject() in c:\C#Apps\github.npgsql.Npgsql.stock\src\Npgsql\NpgsqlDataReader.cs:line 1175 at 
    Npgsql.ForwardsOnlyDataReader.GetNextRowDescription() in c:\C#Apps\github.npgsql.Npgsql.stock\src\Npgsql\NpgsqlDataReader.cs:line 1191    at 
    Npgsql.ForwardsOnlyDataReader.NextResult() in c:\C#Apps\github.npgsql.Npgsql.stock\src\Npgsql\NpgsqlDataReader.cs:line 1377    at 
    Npgsql.NpgsqlCommand.ExecuteNonQuery() in c:\C#Apps\github.npgsql.Npgsql.stock\src\Npgsql\NpgsqlCommand.cs:line523

My system has automatic retry functionality and eventually every record gets inserted into the database, but after many many exceptions when the load is high.

Database is PostgreSQL 9.3 on a CentOS 6 server and client is C# .NET using Npgsql driver.

Table:

CREATE TABLE evidence
(
  id uuid NOT NULL,
  notification_id uuid NOT NULL,
  feedback character varying(200),
  result character varying(20),
  trigger_action_type character varying(200),
  trigger_action_id uuid,
  data_type integer NOT NULL,
  data bytea,
  name character varying(30),
  CONSTRAINT pk_evidence PRIMARY KEY (id)
);

Trigger:

CREATE TRIGGER evidence_move_to_partition_tables
  BEFORE INSERT
  ON evidence
  FOR EACH ROW
  EXECUTE PROCEDURE partition_evidence_by_month();

Trigger Function:

CREATE OR REPLACE FUNCTION partition_evidence_by_month()
  RETURNS trigger AS
$BODY$
DECLARE 
    _notification_id uuid;
    _raised_local_time timestamp without time zone;
    _table_name character varying(35);
    _start_date timestamp without time zone;
    _end_date timestamp without time zone;
    _table_space character varying(50) := 'ls_tablespace2';
    _query text;
BEGIN
    _notification_id := NEW.notification_id;
    SELECT raised_local_time FROM notifications WHERE id=_notification_id INTO _raised_local_time;
    _start_date := date_trunc('month', _raised_local_time);
    _end_date   := _start_date + '1 month'::interval;
    _table_name := 'evidence-' || to_char(_start_date, 'YYYY-MM');

    -- check to see if table already exists
    PERFORM 1
    FROM   pg_catalog.pg_class c
    JOIN   pg_catalog.pg_namespace n ON n.oid = c.relnamespace
    WHERE  c.relkind = 'r'
    AND    c.relname = _table_name
    AND    n.nspname = 'public';

    -- if the table doesn't exist, then create it now
    IF NOT FOUND THEN
        -- create partition table
        _query := 'CREATE TABLE public.' || quote_ident(_table_name) || ' ( ) INHERITS (public.evidence)';
        EXECUTE _query;
        -- alter owner
        --EXECUTE 'ALTER TABLE public.' || quote_ident(_table_name) || ' OWNER TO postgres';
        -- add index
        --EXECUTE 'ALTER TABLE public.' || quote_ident(_table_name) || ' ADD PRIMARY KEY (id)';
    END IF;
    -- move the data to the partition table
    EXECUTE 'INSERT INTO public.' || quote_ident(_table_name) || ' VALUES ($1.*)' USING NEW;
    RETURN NULL;
END;
$BODY$ LANGUAGE plpgsql VOLATILE COST 100;

Calling Code:

using (var cmd = db.CreateCommand())
{
    cmd.CommandText = @"INSERT INTO evidence   
    (id, notification_id, feedback, result, trigger_action_type, 
     trigger_action_id, data_type, data, name) 
    VALUES (@id,@nid,@feedback,@result,@tat,@taid,@dt,@data,@name)";
    cmd.Parameters.AddWithValue("@id", evItem.ID);
    cmd.Parameters.AddWithValue("@nid", evItem.NotificationID);
    cmd.Parameters.AddWithValue("@feedback", evItem.Feedback);
    cmd.Parameters.AddWithValue("@result", evItem.Result);
    cmd.Parameters.AddWithValue("@tat", evItem.TriggerActionType);
    cmd.Parameters.AddWithValue("@taid", evItem.TriggerActionID);
    cmd.Parameters.AddWithValue("@dt", (int)evItem.DataType);
    cmd.Parameters.AddWithValue("@data", evItem.Data);
    cmd.Parameters.AddWithValue("@name", evItem.Name);
    cmd.ExecuteNonQuery();
}

Why would this bizarre error appear only when the system is under load? What can I do to prevent it happening?

Thanks!