求助:Debug Assertion Failed!

程序在执行时出现下面对话框:
Debug Assertion Failed!
Program:E:\ISODTATA\21.EXE
File:fgets.c
Line:60
Expression:str!=NULL
for information on how your program can cause an assertion failure see
the visual c++ documentation on asserts;
如果单击"忽略",则出现下面对话框:
"0x77f821e1"指令引用的"0x0000004"的内存不能为"read"!
希望知道该错误是怎么导致的师长们,能指点迷津!!!!