Fseek() debug assertion failed

Most of the time fseek works smooth, but on some occasion, fseek is crashing application. It will be helpful if one can help finding the bug.
pfile is global pointer variable.

Code:

---

FILE* pfile;

void WriteToFile(char *text)
{       
        long filesize;
        if (pfile) {
                fprintf(pfile,"%s", text);
                fflush(pfile);
        }
        fseek (pfile, 0, SEEK_END);
        filesize=ftell (pfile);
        if ( (long)LOG_FILE_SIZE < filesize) {
                fclose(pfile);
                pfile = NULL;
                CreateLogFile();
        }
}

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This is a C question and not a C++ question.

However without knowing the how WriteToFile is called, what you have done before that and so on it is hard to tell...do you check that a file is properly opened before calling WriteToFile (I see you do a check in the function but that is only done before writing to the file, you may attempt to seek using an invalid file handle)?