POI/HSSF issue populating merged cells in same row

i have the following code:

row# = 2;
hssfrow = HSSFSheet_createRow(hssfsheet : row#);
mergerf = row#;
mergert = row#;
mergecf = 4;
mergect = 9;
hssf_merge( hssfsheet : mergerf : mergecf : mergert : mergect );
hssf_text( hssfrow : 4: 'Merged cells 1' : headstyle2);

mergerf = row#;
mergert = row#;
mergecf = 10;
mergect = 15;
hssf_merge( hssfsheet : mergerf : mergecf : mergert : mergect );
hssf_text( hssfrow : 5: 'merged cells 2' : headstyle2);

When i look at the spreadsheet, the merge worked, but only the first merged cell has data ("Merged cell 1"). The second merged cell is blank.

Any ideas?