"LoadError: MethodError: Cannot convert an object of type Expr to an object of type Symbol" when using @bounce on multiline function definitions

[uribalb]

When I execute this, for example (Jupyter Notebook):

@bounce function pair(n) 
    return n > 1 ? pair(n - 2)  : 1 - n
end

I get the following error:

LoadError: MethodError: Cannot `convert` an object of type Expr to an object of type Symbol
Closest candidates are:
  convert(::Type{T}, !Matched::T) where T at essentials.jl:171
  Symbol(::Any...) at strings/basic.jl:227
in expression starting at In[136]:1
Stacktrace:
 [1] push!(::Array{Symbol,1}, ::Expr) at .\array.jl:934
 [2] (::Lazy.var"#17#18"{Array{Symbol,1}})(::Expr) at C:\Users\username\.julia\packages\Lazy\9Xnd3\src\tail.jl:128
 [3] macro expansion at C:\Users\username\.julia\packages\Lazy\9Xnd3\src\tail.jl:44 [inlined]
 [4] macro expansion at C:\Users\username\.julia\packages\MacroTools\X77lQ\src\match\macro.jl:18 [inlined]
 [5] lastcalls(::Lazy.var"#17#18"{Array{Symbol,1}}, ::Expr) at C:\Users\username\.julia\packages\Lazy\9Xnd3\src\tail.jl:7
 [6] macro expansion at C:\Users\username\.julia\packages\Lazy\9Xnd3\src\tail.jl:44 [inlined]
 [7] macro expansion at C:\Users\username\.julia\packages\MacroTools\X77lQ\src\match\macro.jl:18 [inlined]
 [8] lastcalls(::Lazy.var"#17#18"{Array{Symbol,1}}, ::Expr) at C:\Users\username\.julia\packages\Lazy\9Xnd3\src\tail.jl:7
 [9] (::Lazy.var"#13#14"{Lazy.var"#17#18"{Array{Symbol,1}}})(::Expr) at C:\Users\username\.julia\packages\Lazy\9Xnd3\src\tail.jl:22
 [10] walk(::Expr, ::Function, ::Lazy.var"#13#14"{Lazy.var"#17#18"{Array{Symbol,1}}}) at C:\Users\username\.julia\packages\MacroTools\X77lQ\src\utils.jl:105
 [11] postwalk(::Function, ::Expr) at C:\Users\username\.julia\packages\MacroTools\X77lQ\src\utils.jl:114
 [12] retcalls(::Function, ::Expr) at C:\Users\username\.julia\packages\Lazy\9Xnd3\src\tail.jl:21
 [13] tailcalls(::Function, ::Expr) at C:\Users\username\.julia\packages\Lazy\9Xnd3\src\tail.jl:26
 [14] @bounce(::LineNumberNode, ::Module, ::Any) at C:\Users\username\.julia\packages\Lazy\9Xnd3\src\tail.jl:128
 [15] include_string(::Function, ::Module, ::String, ::String) at .\loading.jl:1091

I suspected that it had something to do with LineNumberNode arguments within the expression, so I decided to prettify the expression in the macro definition and the problem went away:
(at src/tail.jl)

macro bounce(def)
  def = macroexpand(@__MODULE__, prettify(def))
  @assert isdef(def)
  @assert isexpr(def.args[1].args[1], Symbol) # TODO: handle f{T}() = ... 

Still not sure what happened there...