halt with readdir

Pgm has worked for many months. New files added to dir each week. 
Now gets thru 27,219 files in directory and then fails on next file.
Message ID . . . . . . . . . :   CPE3450                                     
Message file . . . . . . . . :   QCPFMSG                                     
  Library  . . . . . . . . . :     QSYS                                      
                                                                             
Message . . . . :   Descriptor not valid.                                    
Cause . . . . . :   A file descriptor argument was out of range, referred to 
  file that was not open, or a read or write request was made to a file that 
  is not open for that operation.                                            

I can see this next file thru Windows Explorer, and actually open file via 
Adobe pdf
When halts - dump option dumps the clp that called the rpg.
How can I bulletproof the below code, or determine whats wrong.
I copied Scott's sample code to put this together.   

C* Open directory                                                  
C                   eval      returnDir = opendir(%addr(FullName)) 
C* Terminate if error occurred when opening directory              
C                   if        ReturnDir = *Null                    
C                   eval      *inlr = *on                          
C                   return                                         
C                   endif                                          
C                   dou       PtrToEntry = *null                   
C* Read next directory entry                                    
C                   eval      PtrToEntry = readdir(ReturnDir)   
C* error?                                                       
C                   if        PtrToEntry = *null                
c                   callp     EscErrno(errno)                   
c                   endif                                       
C* Directory entry name is in field d_name                      
C                   if        PtrToEntry <> *Null               
C                   eval      DirEntry = RtnEntry               
C* Get directory entry name                                     
C                   eval      EntryName = %str(%addr(d_name))   
C* Determine object type of entry                               
C                   eval      EntryPath = %trim(DirName) + '/'  
C                             + %trimr(EntryName) + Null        
C                   eval      ReturnInt = lstat(%addr(EntryPath)
C                                            : %addr(StatDS))   
c*                                                
C                   if        st_objtype = '*STMF'
... more code
C                   endif    
C                   endif    
C                   enddo    

jim franz