# Invalid Shapes error on creating ODEProblem from MTK system

**URL:** https://discourse.julialang.org/t/invalid-shapes-error-on-creating-odeproblem-from-mtk-system/137671
**Category:** Modelling & Simulations
**Created:** [June 17, 2026, 8:02pm UTC](https://discourse.julialang.org/t/invalid-shapes-error-on-creating-odeproblem-from-mtk-system/137671 "2026-06-17T20:02:42Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![mark.garnett](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@mark.garnett](https://discourse.julialang.org/u/mark.garnett)
#### Post date: [June 17, 2026, 8:02pm UTC](https://discourse.julialang.org/t/invalid-shapes-error-on-creating-odeproblem-from-mtk-system/137671/1 "2026-06-17T20:02:42Z")

</div>

I have been running into this problem in many situations and am at a loss to figure it out. It always seems to occur with ifelse() statements, abs(), and the like. Unfortunately I have to have a lot of these types of statements to handle bi-directional fluid flow. I should say I am using stream variables with instream(). You often have to calculate fluid properties for each potential flow direction.

The common thread to all of them is the

```julia-auto
SymbolicUtils.Unknown(2))

```

argument to whatever function that errors out.

I am working on finding a MWE but it is very difficult as it almost seems to be a “Heisenbug”-- it seems to occur inconsistently, depending on what exactly I have run, whether I have restarted the REPL.

I have had some success using the @register\_symbolic macro to hide these conditional like statements.

Upon creating and ODEProblem

```julia-auto
prob = ODEProblem(sys, u0, (0.0, sim_time); guesses=new_guess, use_scc=false)

```

I sometimes get an error of this form:

```julia-auto
ERROR: ArgumentError: Invalid shapes for <=: (UnitRange{Int64}[], SymbolicUtils.Unknown(2)).
Stacktrace:
  [1] _throw_array(::Function, ::SymbolicUtils.SmallVec{UnitRange{Int64}, Vector{UnitRange{Int64}}}, ::Vararg{Any})
    @ SymbolicUtils C:\Users\mark.garnett\.julia\packages\SymbolicUtils\kFlic\src\methods.jl:276
  [2] promote_shape(::typeof(<=), sha::Union{…}, shb::Union{…})
    @ SymbolicUtils C:\Users\mark.garnett\.julia\packages\SymbolicUtils\kFlic\src\methods.jl:589
  [3] maketerm(::Type{…}, f::Any, args::SymbolicUtils.SmallVec{…}, metadata::Nothing; type::Any)
    @ SymbolicUtils C:\Users\mark.garnett\.julia\packages\SymbolicUtils\kFlic\src\terminterface.jl:332
  [4] maketerm(::Type{…}, f::Any, args::SymbolicUtils.SmallVec{…}, metadata::Nothing)
    @ SymbolicUtils C:\Users\mark.garnett\.julia\packages\SymbolicUtils\kFlic\src\terminterface.jl:259
  [5] combine_fold(::Type{…}, op::Any, args::SymbolicUtils.SmallVec{…}, meta::Union{…}, can_fold::Bool)
    @ SymbolicUtils C:\Users\mark.garnett\.julia\packages\SymbolicUtils\kFlic\src\substitute.jl:222
  [6] (::SymbolicUtils.DefaultSubstituter{…})(ex::SymbolicUtils.BasicSymbolicImpl.var"typeof(BasicSymbolicImpl)"{…})
    @ SymbolicUtils C:\Users\mark.garnett\.julia\packages\SymbolicUtils\kFlic\src\substitute.jl:196
  [7] (::SymbolicUtils.DefaultSubstituter{…})(ex::SymbolicUtils.BasicSymbolicImpl.var"typeof(BasicSymbolicImpl)"{…}) (repeats 2 times)
    @ SymbolicUtils C:\Users\mark.garnett\.julia\packages\SymbolicUtils\kFlic\src\substitute.jl:180
  [8] (::Symbolics.FixpointSubstituter{…})(ex::SymbolicUtils.BasicSymbolicImpl.var"typeof(BasicSymbolicImpl)"{…})
    @ Symbolics C:\Users\mark.garnett\.julia\packages\Symbolics\pC2pn\src\variable.jl:437
  [9] maybe_build_initialization_problem(sys::System, iip::Bool, op::ModelingToolkitBase.AtomicArrayDict{…}, t::Float64, guesses::Dict{…}; time_dependent_init::Bool, u0_constructor::Function, p_constructor::Function, floatT::Type, initialization_eqs::Vector{…}, use_scc::Bool, eval_expression::Bool, eval_module::Module, missing_guess_value::ModelingToolkitBase.MissingGuessValue.var"typeof(MissingGuessValue)", implicit_dae::Bool, is_steadystateprob::Bool, expression::Type, kwargs::@Kwargs{…})
    @ ModelingToolkitBase C:\Users\mark.garnett\.julia\packages\ModelingToolkitBase\Rcink\src\systems\problem_utils.jl:1382
 [10] process_SciMLProblem(constructor::Type, sys::System, op::Vector{…}; build_initializeprob::Bool, implicit_dae::Bool, t::Float64, guesses::Dict{…}, warn_initialize_determined::Bool, initialization_eqs::Vector{…}, eval_expression::Bool, eval_module::Module, fully_determined::Nothing, check_initialization_units::Bool, u0_eltype::Nothing, tofloat::Bool, u0_constructor::typeof(identity), p_constructor::typeof(identity), check_length::Bool, symbolic_u0::Bool, warn_cyclic_dependency::Bool, circular_dependency_max_cycle_length::Int64, circular_dependency_max_cycles::Int64, initsys_mtkcompile_kwargs::@NamedTuple{}, substitution_limit::Int64, use_scc::Bool, time_dependent_init::Bool, algebraic_only::Bool, missing_guess_value::ModelingToolkitBase.MissingGuessValue.var"typeof(MissingGuessValue)", allow_incomplete::Bool, is_initializeprob::Bool, is_steadystateprob::Bool, return_operating_point::Bool, kwargs::@Kwargs{…})
    @ ModelingToolkitBase C:\Users\mark.garnett\.julia\packages\ModelingToolkitBase\Rcink\src\systems\problem_utils.jl:1603
 [11] (ODEProblem{…})(sys::System, op::Any, tspan::Tuple{…}; callback::Any, check_length::Bool, eval_expression::Bool, expression::Type, eval_module::Module, check_compatibility::Bool, _skip_events::Bool, kwargs::@Kwargs{…})
    @ ModelingToolkitBase C:\Users\mark.garnett\.julia\packages\ModelingToolkitBase\Rcink\src\problems\odeproblem.jl:111
 [12] ODEProblem
    @ C:\Users\mark.garnett\.julia\packages\ModelingToolkitBase\Rcink\src\problems\odeproblem.jl:101 [inlined]
 [13] ODEProblem
    @ .\none:-1 [inlined]
 [14] #ODEProblem#792
    @ .\none:-1 [inlined]

```

Has anyone run into this? Using MTK v11.26.0

---

<div class="post-metadata">

### Author: ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [June 18, 2026, 8:33am UTC](https://discourse.julialang.org/t/invalid-shapes-error-on-creating-odeproblem-from-mtk-system/137671/2 "2026-06-18T08:33:50Z")

</div>

Open an issue in the MTK repo for this.

---

<div class="post-metadata">

### Author: ![mark.garnett](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@mark.garnett](https://discourse.julialang.org/u/mark.garnett)
#### Post date: [June 18, 2026, 8:00pm UTC](https://discourse.julialang.org/t/invalid-shapes-error-on-creating-odeproblem-from-mtk-system/137671/3 "2026-06-18T20:00:21Z")

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Thank you Chris I have opened an issue [https://github.com/SciML/ModelingToolkit.jl/issues/4642](https://github.com/SciML/ModelingToolkit.jl/issues/4642)