Could not read any samples from GATK depthcoverage file.

I have two sets of depth coverage file from GATK 2.5-2 and would like to merge them. however, I am facing the following error.

ERROR: Could not read any samples from /medpop/mpg-psrl/Parabase/Projects/XHMM/group3.DATA.sample_summary [expected suffix in header is: _mean_cvg]

the header of the file looks like as following, ...'

        sample_id       total   mean    granular_third_quartile granular_median granular_first_quartile %_bases_above_15
        c1121LEVa_out   2889209608      61.43   97      54      27      86.2
        c1015EAGa       3376953408      71.80   109     70      40      91.5
        c1135QXOa       2104061838      44.73   63      38      21      82.6
        c1131CARa_out   3739532095      79.50   129     81      42      90.8
        c1119THOa_out   3122424859      66.38   107     60      30      87.7
        c1031SMIa       3004514137      63.88   99      58      29      87.1
        m1015HAYa.40286 2127170066      45.22   64      43      24      85.4
        c1017PRIa_out   3263380845      69.38   113     64      33      88.7
        Total   23627246856     502.33  N/A     N/A     N/A

I appreciate your comment,