Tophat's accepted_hits.sam reports all the ISIZEs equal zero

[ruping]

Hi, I run tophat for pair-end reads, but the results report all the ISIZE equal 0, which is quite confusing:


read1 99 chr1 26773653 255 36M = 26773726 0 GGGATCCCACCCTGGGGACCCCCACGATTGGCCACC ;<A555?<;?6C@D-5@...
read2 147 chr1 26773726 255 36M = 26773653 0 TCACCTCTCTCAGAGCATCTGGCTGTTTAGCAGAAC 3C0@C=C@?-C=?A?C@...
read3 137 chr1 27542972 255 36M * 0 0 GCCTATCCCCATTGGTGTGCATATGCTGTGTGCAGG 3,?33807;*;2;;,>B;B))*756...


based on the definition of ISIZE, if the two reads in a pair are mapped to the same reference, ISIZE should be calculated, why it's all zero here?

Thanks in advance for help.