String Index out of range Error

Hi all

I have an error "String index out of range: -4". What does this error mean??
The error seems to be in this line of code "String group = gstr[i].substring(3,endposition);"
There are 21 groups that must be added to Vector vsap, but on the 20th time in the loop, it displays that error.

Here is my code
try{
  //get all grps in portal, return in string[]
gstr = igf.findGroups(igf.getSearchGroup(),0);

//sort the string
Arrays.sort(gstr, String.CASE_INSENSITIVE_ORDER);

if(gstr != null){
response.write("<script>alert(\"gstr:"+gstr.length+"\")</script>");
      
for(int i=0; i<gstr.length; i++){
response.write("<script>alert(\"i :"+i+"\")</script>");
endposition = gstr[i].indexOf(',');
String group = gstr[i].substring(3,endposition);

//if group starts with sapportal groups , add it to vector
if(group.startsWith("SAPPortal")){
response.write("<script>alert(\"grp:"+group+"\")</script>");
vsap.addElement(group);
}                              
}      
setVSap(vsap);
}//if gstr != null
else{
response.write("<script>alert(\"String [] grps is null\")</script>");
}      
                  
}//end try
catch(Exception e){
response.write("<script>alert(\"issues:"+e.getMessage()+"\")</script>");
}

Thanx
SB