```
using JuMP, Ipopt, Distances
model = Model(Ipopt.Optimizer)
@variable(model, a)
@variable(model, b)
euclidean(a, b)
```
Results in:
```
ERROR: MethodError: no method matching length(::VariableRef)

Closest candidates are:
  length(::Union{Base.KeySet, Base.ValueIterator})
   @ Base abstractdict.jl:58
  length(::Union{LinearAlgebra.Adjoint{T, S}, LinearAlgebra.Transpose{T, S}} where {T, S})
   @ LinearAlgebra /builddirs/julia/usr/share/julia/stdlib/v1.9/LinearAlgebra/src/adjtrans.jl:295
  length(::Union{SparseArrays.FixedSparseVector{Tv, Ti}, SparseArrays.SparseVector{Tv, Ti}} where {Tv, Ti})
   @ SparseArrays /builddirs/julia/usr/share/julia/stdlib/v1.9/SparseArrays/src/sparsevector.jl:95
  ...

```

```
julia> euclidean(a, b)
ERROR: MethodError: no method matching oneunit(::Type{Any})

Closest candidates are:
  oneunit(::Type{Union{Missing, T}}) where T
   @ Base missing.jl:105
  oneunit(::Type{T}) where T
   @ Base number.jl:370
  oneunit(::T) where T
   @ Base number.jl:369
  ...

Stacktrace:
 [1] oneunit(#unused#::Type{Any})
   @ Base ./missing.jl:106
 [2] _eval_start(d::Euclidean, #unused#::Type{Any}, #unused#::Type{Any}, #unused#::Nothing)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:320
 [3] _eval_start(d::Euclidean, #unused#::Type{Any}, #unused#::Type{Any})
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:318
 [4] eval_start(d::Euclidean, a::VariableRef, b::VariableRef)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:317
 [5] _evaluate(d::Euclidean, a::VariableRef, b::VariableRef, #unused#::Nothing)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:236
 [6] (::Euclidean)(a::VariableRef, b::VariableRef)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:328
 [7] top-level scope
   @ REPL[10]:1

```

```
julia> euclidean(a, b)
ERROR: MethodError: no method matching VariableRef(::AffExpr)

Closest candidates are:
  (::Type{GenericVariableRef{T}} where T)(::Any, ::Any)
   @ JuMP ~/.julia/packages/JuMP/h0lrf/src/variables.jl:251
  GenericVariableRef{T}(::ConstraintRef{GenericModel{T}, <:MathOptInterface.ConstraintIndex{MathOptInterface.VariableIndex}}) where T
   @ JuMP ~/.julia/packages/JuMP/h0lrf/src/variables.jl:520
  GenericVariableRef{T}(::GenericModel{T}) where T
   @ JuMP ~/.julia/packages/JuMP/h0lrf/src/variables.jl:494

Stacktrace:
 [1] oneunit(#unused#::Type{VariableRef})
   @ Base ./number.jl:370
 [2] _eval_start(d::Euclidean, #unused#::Type{VariableRef}, #unused#::Type{VariableRef}, #unused#::Nothing)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:320
 [3] _eval_start(d::Euclidean, #unused#::Type{VariableRef}, #unused#::Type{VariableRef})
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:318
 [4] eval_start(d::Euclidean, a::VariableRef, b::VariableRef)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:317
 [5] _evaluate(d::Euclidean, a::VariableRef, b::VariableRef, #unused#::Nothing)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:236
 [6] (::Euclidean)(a::VariableRef, b::VariableRef)
   @ Distances ~/.julia/packages/Distances/PvoXa/src/metrics.jl:328
 [7] top-level scope
   @ REPL[8]:1

```