Why is SymbolName() returning nothing?

In a script designed to loop through every available symbol and log the information available:

int i, j, file_handle;
string InpDirectoryName, InpFileName, Textwrite, mySymbol;
//+------------------------------------------------------------------+
//| Script program start function                                    |
//+------------------------------------------------------------------+
void OnStart()
  {
//---
int MIIds[29]; // 0-28 market information type IDs
int ST;
MIIds[0] = 1;
MIIds[1] = 2;
MIIds[2] = 5;
for(i=3;i<=28; i++)
    MIIds[i] = i+6;
ST = SymbolsTotal(false);
InpDirectoryName = "";
InpFileName = "Test.csv";
Textwrite = "";
file_handle=FileOpen(InpDirectoryName+InpFileName,FILE_READ|FILE_WRITE|FILE_CSV); 
if(file_handle!=INVALID_HANDLE) 
   {
   for(i=1;i<=ST; i++)
      Print(i);

So far, so good, I get a list of Symbol list positions printed. However, on attempting to call the corresponding SymbolName(),

    mySymbol = SymbolName(i, false); 

there is no further print output from the next line:

    Print(mySymbol);

The output being as follows:

0   12:29:56.630    Script test EURUSD,H4: loaded successfully
0   12:29:56.667    test EURUSD,H4: initialized
0   12:29:56.669    test EURUSD,H4: 1
...
0   12:29:56.670    test EURUSD,H4: 68
0   12:29:56.670    test EURUSD,H4: 
0   12:29:56.670    test EURUSD,H4: uninit reason 0
0   12:29:56.676    Script test EURUSD,H4: removed

As you can see, there are no error codes, just a line of empty output before uninitialisation... This is the rest of the code for the sake of completeness, although not currently at issue:

    SymbolSelect(mySymbol,true);
    Textwrite = mySymbol;
    for(j=0;i<=28; j++)
        Textwrite = Textwrite + "," + MarketInfo(mySymbol,j);
    FileWrite(file_handle,Textwrite);
   FileClose(file_handle);
   }
else Print("Operation FileOpen failed, error ",GetLastError()); 
  }

Any thoughts gratefully received