Fgetl error on script

Hello. I have a problem with fgetl function. Specifically i hace the following script...

....
fid=fopen(outfile,'r');

      if fid<0
          error('wrong output file name, Stamatame tin analysi')
      end

      nodeRes =[];
      linkRes = [];
      icntLine = 0;

      while 1
          icntLine=icntLine+1;
          tline=fgetl(fid);        

          if ~ischar(tline), break, else i=i+1; end
  ....

When i press run button, code run successfully for the first repeat of loop. When icntLine=2 script fail with the message

Error using fgets
Invalid file identifier.  Use fopen to generate a valid file identifier.

Error in fgetl (line 34)
[tline,lt] = fgets(fid);

Error in main_prgtry (line 72)
        tline=fgetl(fid);

if i run the fgetl from command window, it runs successfully... Can anybody help me? Thank you

[Geoff Hayes]

giannis - can you show us all of the code for the while block? Are you manipulating the fid anywhere else in the code?