I created a simple model and tried to copy and modify.

```
using JuMP, Clp
m = Model(solver=ClpSolver())
set_A = [(1,2), (2,3)]
@variable(m, x[(i,j) in set_A]>=0)
@objective(m, Max, sum(x[(i,j)] for (i,j) in set_A) )
@constraint(m, x[(1,2)]<=10)
@constraint(m, x[(2,3)]<=10)
solve(m)
@show getvalue(x)
m2 = copy(m)
x2 = copy(x, m2)
@constraint(m2, x2[(1,2)]<=1)
@constraint(m2, x2[(2,3)]<=1)
solve(m2)
@show getvalue(x2)
```

The last line creates an error. When tuples were not used in the variable definition, it works fine.

```
getvalue(x) = x: 1 dimensions:
[(1, 2)] = 10.0
[(2, 3)] = 10.0
ERROR: LoadError:
Stacktrace:
[1] getindex(::ObjectIdDict, ::JuMP.JuMPArray{JuMP.Variable,1,Tuple{Array{Tuple{Int64,Int64},1}}}) at ./associative.jl:363
[2] _map(::Function, ::JuMP.JuMPArray{JuMP.Variable,1,Tuple{Array{Tuple{Int64,Int64},1}}}) at /Users/chkwon/.julia/v0.6/JuMP/src/JuMPContainer.jl:162
[3] getvalue(::JuMP.JuMPArray{JuMP.Variable,1,Tuple{Array{Tuple{Int64,Int64},1}}}) at /Users/chkwon/.julia/v0.6/JuMP/src/JuMPContainer.jl:114
[4] include_from_node1(::String) at ./loading.jl:576
[5] include(::String) at ./sysimg.jl:14SYSTEM: show(lasterr) caused an error
AssertionError("j isa Array{Variable}")
Stacktrace:
[1] getindex(::ObjectIdDict, ::JuMP.JuMPArray{JuMP.Variable,1,Tuple{Array{Tuple{Int64,Int64},1}}}) at ./associative.jl:363
[2] _map(::Function, ::JuMP.JuMPArray{JuMP.Variable,1,Tuple{Array{Tuple{Int64,Int64},1}}}) at /Users/chkwon/.julia/v0.6/JuMP/src/JuMPContainer.jl:162
[3] getvalue(::JuMP.JuMPArray{JuMP.Variable,1,Tuple{Array{Tuple{Int64,Int64},1}}}) at /Users/chkwon/.julia/v0.6/JuMP/src/JuMPContainer.jl:114
[4] include_from_node1(::String) at ./loading.jl:576
[5] include(::String) at ./sysimg.jl:14
[6] eval(::Module, ::Any) at ./boot.jl:235
[7] eval_user_input(::Any, ::Base.REPL.REPLBackend) at ./REPL.jl:66
[8] macro expansion at ./REPL.jl:97 [inlined]
[9] (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at ./event.jl:73
```

Is this a bug?