Run-Time Check Failure #2 - Stack around the variable 'x' was corrupted

String GetVariableName(CString symbol, CString filepath)
{
    char acLine[512];
    char acPreviousLine[512];
    CString csFile;
    FILE *fp;

    csFile.Format("%svariables.txt", filepath);

    fp = fopen(csFile, "r");

    if (! fp)
    	return("");

    for (;;)
    {
    	strcpy(acPreviousLine, acLine);

    	// NULL means we are out of lines in the file.
    	if (myfgets(acLine, 511, fp) == NULL)
    		break;

        // "END" indicates end of file
    	if (! strcmp(acLine, "END"))
    		break;

    	if (! strcmp(acLine, csVarSymbol))
    	{
    		// Previous line should be variable name
    		fclose(fp);

            // Following line results in Check Failure while in Debug mode
    		return(acPreviousLine);
    	}
    }
    fclose(fp);
    return("");
}