I got an error "Error termination. Backtrace."

After I compiled a file by gfortran, I tried to run the exe file. And I got an error following;

At line 94 of file eee.f (unit = 21, file = 'fort.21')
Fortran runtime error: End of file

Error termination. Backtrace:
#0 0x7f035c6dbeea
#1 0x7f035c6dca45
#2 0x7f035c6dd1fc
#3 0x7f035c7a92a3
#4 0x7f035c7a97f7
#5 0x7f035c7a69b3
#6 0x7f035c7ab644
#7 0x4790fc
#8 0x4010fc
#9 0x7f035bae1bf6
#10 0x40112c
#11 0xffffffffffffffff

Also, the line 94 of file eee.f is
read (21,'(a200)')qstrana1

The source code files are written by fortran 77, but I have just used gfortran compiler because of difficulty of download fortran 77 compiler.

How can I solve this problem? And could you guys let me know when backtrace error occurs?

Is it caused by compiling procedure? or the lack of input file?

I'm a newbie in Linux, so I'm not sure I give an enough information to figure out this problem. I'll upload more information if you want.