Attempted to print `nothing` while stepping through `Flux.back!()`

Example:

using Flux
W = param(randn(10,10))
y = sum(W * randn(10))

Attempting to @enter the Flux.back!(y) works, but halfway through I reliably hit errors when attempting to step within (Core.kwfunc):

julia> @enter Flux.back!(y)
In #back!#18(once, , x) at /home/staticfloat/.julia/packages/Flux/T3PhK/src/tracker/lib/real.jl:14
14      isinf(x) && error("Loss is Inf")
15      isnan(x) && error("Loss is NaN")
16      return back!(x, 1, once = once)

About to run: (isinf)(-24.445557315016536 (tracked))
1|debug > n
In #back!#18(once, , x) at /home/staticfloat/.julia/packages/Flux/T3PhK/src/tracker/lib/real.jl:14
14      isinf(x) && error("Loss is Inf")
15      isnan(x) && error("Loss is NaN")
16      return back!(x, 1, once = once)
17  end

About to run: (isnan)(-24.445557315016536 (tracked))
1|debug > n
In #back!#18(once, , x) at /home/staticfloat/.julia/packages/Flux/T3PhK/src/tracker/lib/real.jl:14
14      isinf(x) && error("Loss is Inf")
15      isnan(x) && error("Loss is NaN")
16      return back!(x, 1, once = once)
17  end
18

About to run: (Core.apply_type)(NamedTuple, (:once,))
1|debug > si
In #back!#18(once, , x) at /home/staticfloat/.julia/packages/Flux/T3PhK/src/tracker/lib/real.jl:14
14      isinf(x) && error("Loss is Inf")
15      isnan(x) && error("Loss is NaN")
16      return back!(x, 1, once = once)
17  end
18

About to run: (tuple)(true)
1|debug > si
In #back!#18(once, , x) at /home/staticfloat/.julia/packages/Flux/T3PhK/src/tracker/lib/real.jl:14
14      isinf(x) && error("Loss is Inf")
15      isnan(x) && error("Loss is NaN")
16      return back!(x, 1, once = once)
17  end
18

About to run: (NamedTuple{(:once,),T} where T<:Tuple)((true,))
1|debug > si
In Type(args) at boot.jl:547
547   NamedTuple{names}(args::Tuple) where {names} = NamedTuple{names,typeof(args)}(args)
548
549   using .Intrinsics: sle_int, add_int

About to run: $(Expr(:static_parameter, 1))
1|debug > finish
In #back!#18(once, , x) at /home/staticfloat/.julia/packages/Flux/T3PhK/src/tracker/lib/real.jl:14
14      isinf(x) && error("Loss is Inf")
15      isnan(x) && error("Loss is NaN")
16      return back!(x, 1, once = once)
17  end
18

About to run: (Core.kwfunc)(Flux.Tracker.back!)
1|debug > si
ERROR: ArgumentError: `nothing` should not be printed; use `show`, `repr`, or custom output instead.
Stacktrace:
 [1] print(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Nothing) at ./show.jl:566
 [2] print_next_state(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::DebuggerFramework.DebuggerState, ::ASTInterpreter2.JuliaStackFrame) at /home/staticfloat/.julia/dev/ASTInterpreter2/src/ASTInterpreter2.jl:180
 [3] print_status(::Base.TTY, ::DebuggerFramework.DebuggerState, ::ASTInterpreter2.JuliaStackFrame) at /home/staticfloat/.julia/dev/DebuggerFramework/src/DebuggerFramework.jl:209
 [4] print_status(::Base.TTY, ::DebuggerFramework.DebuggerState) at /home/staticfloat/.julia/dev/DebuggerFramework/src/DebuggerFramework.jl:181
 [5] (::getfield(DebuggerFramework, Symbol("##4#8")){REPL.Terminals.TTYTerminal,DebuggerFramework.DebuggerState,REPL.LineEdit.Prompt})(::REPL.LineEdit.MIState, ::Base.GenericIOBuffer{Array{UInt8,1}}, ::Bool) at /home/staticfloat/.julia/dev/DebuggerFramework/src/DebuggerFramework.jl:290
 [6] #invokelatest#1 at ./essentials.jl:754 [inlined]
 [7] invokelatest at ./essentials.jl:753 [inlined]
 [8] run_interface(::REPL.Terminals.TextTerminal, ::REPL.LineEdit.ModalInterface, ::REPL.LineEdit.MIState) at /home/staticfloat/tmp/julia-build/julia-master/usr/share/julia/stdlib/v1.2/REPL/src/LineEdit.jl:2273
 [9] run_interface at /home/staticfloat/tmp/julia-build/julia-master/usr/share/julia/stdlib/v1.2/REPL/src/LineEdit.jl:2267 [inlined]
 [10] RunDebugger(::Array{ASTInterpreter2.JuliaStackFrame,1}, ::REPL.LineEditREPL, ::REPL.Terminals.TTYTerminal) at /home/staticfloat/.julia/dev/DebuggerFramework/src/DebuggerFramework.jl:314
 [11] RunDebugger(::Array{ASTInterpreter2.JuliaStackFrame,1}) at /home/staticfloat/.julia/dev/DebuggerFramework/src/DebuggerFramework.jl:240
 [12] top-level scope at /home/staticfloat/.julia/dev/ASTInterpreter2/src/ASTInterpreter2.jl:478

Reproduce the failure by stepping through the reported Flux.back!(y) example with @enter. Start at ASTInterpreter2/src/ASTInterpreter2.jl:180, where print_next_state prints the debugger state, and inspect the related status path in DebuggerFramework/src/DebuggerFramework.jl:209. Done means stepping through this call no longer raises the nothing printing error.