modelsim仿真出现错误FATAL ERROR while loading design 该怎么解决，谢谢

独自莫凭栏

初学modelsim，仿真时出现问题，FATAL ERROR while loading design
请问是什么原因，怎样解决？
9 x! `; y7 Z2 F# y1 z: z1 C# vsim -vopt work.mul
$ O) ~  X4 X) g' \# ** Note: (vsim-3812) Design is being optimized...
# Loading G:\Modeltech_6.2b\win32/../std.standard
. {+ v' ^9 H3 k) o% R1 x5 }# Loading work.mul(func)
# ** Fatal: (vsim-3347) Port 'in1' is not constrained.
#    Time: 0 ns  Iteration: 0  Instance: /mul File: G:/Modeltech_6.2b/examples/mul.vhd Line: 3
' c: y+ `' L6 k& K# FATAL ERROR while loading design
& a/ G( I2 M: e6 i) W# Error loading design

请大家帮帮忙，谢谢