Inserting NaN or Null values into a text file processed by MySqlBulkLoader

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Hello,

I am creating a text file for MySqlBulkLoader to process. The table is simple:

CREATE TABLE VariableData (
ExperimentID INTEGER UNSIGNED NOT NULL,
VariableID INTEGER UNSIGNED NOT NULL,
ordinal INTEGER UNSIGNED NOT NULL,
value DOUBLE NULL,
PRIMARY KEY(ExperimentID, VariableID, ordinal)
);

Sometimes the 'value' value is NaN. What do I need to write in the text file that BulkLoader processes in order to insert either NaN or a Null into the table? I've tried these two strings:

stmt = idExperimentStr + ',' + idVariableStr + ',' + str(ordinal) + ',NaN\r'
stmt = idExperimentStr + ',' + idVariableStr + ',' + str(ordinal) + ',\r'

but '0' gets written as the 'value' in both statements.