C++ :: Write Data Into Files In A For Loop - Debug Assertion Failed?

In my program I open two files and write data into each, and close them. this is done in a while loop.

int main() {
while() {
myFile = fopen("file1.txt","w");
myFile2 = fopen("file2.txt","w");
// write data into files using fprintf() in a for loop
fclose(myFile);
fclose(myFile2);
}
}

However, after some iterations the fopen() for myFile2 fails.

Debug assertion failed!
Program....
File ......srcfprintf.c
Line 55

Expression (str != NULL)

I have not been ableto locate the bug in the code that is causing the failure.