Warning: Internal error: Variable (l0(t))[6] was marked as being in ..., but was actually zero

[1-Bart-1]

Describe the bug 🐞

I am getting this warning message:

┌ Warning: Internal error: Variable (l0(t))[9] was marked as being in 0 ~ -(stiffness(t))[9] + ifelse((len(t))[9] > (l0(t))[9], (0.7853981633974483stiffness_frac*SymbolicAWEModels.get_e_tether(pset)*(SymbolicAWEModels.get_diameter(psys, 9)^2)) / (len(t))[9], (0.7853981633974483stiffness_frac*SymbolicAWEModels.get_e_tether(pset)*SymbolicAWEModels.get_compression_frac(psys, 9)*(SymbolicAWEModels.get_diameter(psys, 9)^2)) / (len(t))[9]), but was actually zero
└ @ ModelingToolkit.StructuralTransformations ~/.julia/packages/ModelingToolkit/vyvd9/src/structural_transformation/utils.jl:291

Expected behavior

I did not get this warning message before. This came after updating the packages, without changing the code.

Minimal Reproducible Example 👇

using SymbolicAWEModels, KiteUtils, VortexStepMethod, ModelingToolkit

set_data_path("data")
if !ispath("data/settings_ram.yaml")
    SymbolicAWEModels.copy_model_settings()
end

function generate_sys(segments)
    set = Settings("system_ram.yaml")
    set.segments = segments
    set.physical_model = "ram"
    sam = SymbolicAWEModel(set)
    println("Creating sys with $segments segments")
    @time inputs = SymbolicAWEModels.create_sys!(sam, sam.sys_struct; 
        init_va_b=zeros(1,3), prn=false)
    println("Simplifying sys with $segments segments")
    @time sys = mtkcompile(sam.full_sys; inputs)
end

generate_sys(3)
generate_sys(3) # 50 seconds
nothing

Environment (please complete the following information):

• Output of using Pkg; Pkg.status()

Status `~/Tes/Project.toml`
  [90980105] KiteUtils v0.10.15
⌅ [961ee093] ModelingToolkit v10.10.0
  [9c9a347c] SymbolicAWEModels v0.1.3
  [ed3cd733] VortexStepMethod v2.0.0

• Output of using Pkg; Pkg.status(; mode = PKGMODE_MANIFEST)

Status `~/Tes/Manifest.toml`
  [47edcb42] ADTypes v1.15.0
  [621f4979] AbstractFFTs v1.5.0
  [1520ce14] AbstractTrees v0.4.5
  [7d9f7c33] Accessors v0.1.42
  [79e6a3ab] Adapt v4.3.0
  [66dad0bd] AliasTables v1.1.3
  [ec485272] ArnoldiMethod v0.4.0
  [4fba245c] ArrayInterface v7.19.0
  [4c555306] ArrayLayouts v1.11.1
  [69666777] Arrow v2.8.0
  [31f734f8] ArrowTypes v2.3.0
  [c59cac55] AtmosphericModels v0.3.1
  [13072b0f] AxisAlgorithms v1.1.0
  [e2ed5e7c] Bijections v0.2.2
  [c3b6d118] BitIntegers v0.3.5
  [62783981] BitTwiddlingConvenienceFunctions v0.1.6
  [8e7c35d0] BlockArrays v1.7.0
  [70df07ce] BracketingNonlinearSolve v1.3.0
  [2a0fbf3d] CPUSummary v0.2.6
  [336ed68f] CSV v0.10.15
  [d360d2e6] ChainRulesCore v1.25.2
  [fb6a15b2] CloseOpenIntervals v0.1.13
  [5ba52731] CodecLz4 v0.4.6
  [ba30903b] CodecXz v0.7.4
  [944b1d66] CodecZlib v0.7.8
  [6b39b394] CodecZstd v0.8.6
  [3da002f7] ColorTypes v0.12.1
  [5ae59095] Colors v0.13.1
  [861a8166] Combinatorics v1.0.3
  [a80b9123] CommonMark v0.9.1
  [38540f10] CommonSolve v0.2.4
  [bbf7d656] CommonSubexpressions v0.3.1
  [f70d9fcc] CommonWorldInvalidations v1.0.0
  [34da2185] Compat v4.17.0
  [b152e2b5] CompositeTypes v0.1.4
  [a33af91c] CompositionsBase v0.1.2
  [2569d6c7] ConcreteStructs v0.2.3
  [f0e56b4a] ConcurrentUtilities v2.5.0
  [187b0558] ConstructionBase v1.6.0
  [adafc99b] CpuId v0.3.1
  [a8cc5b0e] Crayons v4.1.1
  [9a962f9c] DataAPI v1.16.0
  [864edb3b] DataStructures v0.18.22
  [e2d170a0] DataValueInterfaces v1.0.0
  [3f0dd361] DefaultApplication v1.1.0
  [8bb1440f] DelimitedFiles v1.9.1
  [2b5f629d] DiffEqBase v6.177.2
  [459566f4] DiffEqCallbacks v4.8.0
  [77a26b50] DiffEqNoiseProcess v5.24.1
  [163ba53b] DiffResults v1.1.0
  [b552c78f] DiffRules v1.15.1
⌅ [a0c0ee7d] DifferentiationInterface v0.6.54
  [c1363496] DiscretePIDs v0.1.6
  [8d63f2c5] DispatchDoctor v0.4.26
  [31c24e10] Distributions v0.25.120
  [ffbed154] DocStringExtensions v0.9.5
  [5b8099bc] DomainSets v0.7.15
  [7c1d4256] DynamicPolynomials v0.6.2
  [06fc5a27] DynamicQuantities v1.8.0
  [4e289a0a] EnumX v1.0.5
  [f151be2c] EnzymeCore v0.8.12
  [e2ba6199] ExprTools v0.1.10
  [55351af7] ExproniconLite v0.10.14
  [7a1cc6ca] FFTW v1.9.0
  [7034ab61] FastBroadcast v0.3.5
  [9aa1b823] FastClosures v0.3.2
  [a4df4552] FastPower v1.1.3
  [5789e2e9] FileIO v1.17.0
  [48062228] FilePathsBase v0.9.24
  [1a297f60] FillArrays v1.13.0
  [64ca27bc] FindFirstFunctions v1.4.1
  [6a86dc24] FiniteDiff v2.27.0
  [53c48c17] FixedPointNumbers v0.8.5
  [1fa38f19] Format v1.3.7
⌅ [f6369f11] ForwardDiff v0.10.38
  [069b7b12] FunctionWrappers v1.1.3
  [77dc65aa] FunctionWrappersWrappers v0.1.3
  [d9f16b24] Functors v0.5.2
  [46192b85] GPUArraysCore v0.2.0
  [c27321d9] Glob v1.3.1
  [86223c79] Graphs v1.13.0
  [34004b35] HypergeometricFunctions v0.3.28
  [615f187c] IfElse v0.1.1
  [3263718b] ImplicitDiscreteSolve v0.1.2
  [d25df0c9] Inflate v0.1.5
  [842dd82b] InlineStrings v1.4.4
  [18e54dd8] IntegerMathUtils v0.1.3
⌅ [a98d9a8b] Interpolations v0.15.1
  [8197267c] IntervalSets v0.7.11
  [3587e190] InverseFunctions v0.1.17
  [92d709cd] IrrationalConstants v0.2.4
  [82899510] IteratorInterfaceExtensions v1.0.0
  [692b3bcd] JLLWrappers v1.7.0
  [ae98c720] Jieko v0.2.1
  [98e50ef6] JuliaFormatter v2.1.2
⌅ [70703baa] JuliaSyntax v0.4.10
  [ccbc3e58] JumpProcesses v9.16.1
  [90980105] KiteUtils v0.10.15
  [ba0b0d4f] Krylov v0.10.1
  [b964fa9f] LaTeXStrings v1.4.0
  [23fbe1c1] Latexify v0.16.8
  [10f19ff3] LayoutPointers v0.1.17
  [5078a376] LazyArrays v2.6.1
  [87fe0de2] LineSearch v0.1.4
  [d3d80556] LineSearches v7.4.0
  [7ed4a6bd] LinearSolve v3.20.0
  [2ab3a3ac] LogExpFunctions v0.3.29
  [d8e11817] MLStyle v0.4.17
  [1914dd2f] MacroTools v0.5.16
  [d125e4d3] ManualMemory v0.1.8
  [bb5d69b7] MaybeInplace v0.1.4
  [442fdcdd] Measures v0.3.2
  [ebf956a0] MeshGrid v1.0.3
  [e1d29d7a] Missings v1.2.0
  [78c3b35d] Mocking v0.8.1
⌅ [961ee093] ModelingToolkit v10.10.0
  [2e0e35c7] Moshi v0.3.7
  [46d2c3a1] MuladdMacro v0.2.4
  [102ac46a] MultivariatePolynomials v0.5.9
  [d8a4904e] MutableArithmetics v1.6.4
  [d41bc354] NLSolversBase v7.10.0
  [15e1cf62] NPZ v0.4.3
  [77ba4419] NaNMath v1.1.3
  [8913a72c] NonlinearSolve v4.10.0
  [be0214bd] NonlinearSolveBase v1.13.0
  [5959db7a] NonlinearSolveFirstOrder v1.5.0
  [9a2c21bd] NonlinearSolveQuasiNewton v1.6.0
  [26075421] NonlinearSolveSpectralMethods v1.2.0
  [6fe1bfb0] OffsetArrays v1.17.0
  [429524aa] Optim v1.13.2
  [bac558e1] OrderedCollections v1.8.1
  [6ad6398a] OrdinaryDiffEqBDF v1.6.0
  [bbf590c4] OrdinaryDiffEqCore v1.26.2
  [4302a76b] OrdinaryDiffEqDifferentiation v1.10.1
  [127b3ac7] OrdinaryDiffEqNonlinearSolve v1.11.0
  [2d112036] OrdinaryDiffEqSDIRK v1.3.0
  [90014a1f] PDMats v0.11.35
  [d96e819e] Parameters v0.12.3
  [69de0a69] Parsers v2.8.3
  [e409e4f3] PoissonRandom v0.4.5
  [f517fe37] Polyester v0.7.18
  [1d0040c9] PolyesterWeave v0.2.2
  [2dfb63ee] PooledArrays v1.4.3
  [85a6dd25] PositiveFactorizations v0.2.4
  [d236fae5] PreallocationTools v0.4.29
⌅ [aea7be01] PrecompileTools v1.2.1
  [21216c6a] Preferences v1.4.3
  [08abe8d2] PrettyTables v2.4.0
  [27ebfcd6] Primes v0.5.7
  [43287f4e] PtrArrays v1.3.0
  [1fd47b50] QuadGK v2.11.2
  [94ee1d12] Quaternions v0.7.6
  [74087812] Random123 v1.7.1
  [e6cf234a] RandomNumbers v1.6.0
  [c84ed2f1] Ratios v0.4.5
  [c1ae055f] RealDot v0.1.0
  [3cdcf5f2] RecipesBase v1.3.4
  [731186ca] RecursiveArrayTools v3.34.1
  [189a3867] Reexport v1.2.2
  [74f56ac7] ReferenceFrameRotations v3.1.1
  [ae029012] Requires v1.3.1
  [ae5879a3] ResettableStacks v1.1.1
  [79098fc4] Rmath v0.8.0
  [6038ab10] Rotations v1.7.1
  [7e49a35a] RuntimeGeneratedFunctions v0.5.15
  [9dfe8606] SCCNonlinearSolve v1.4.0
  [94e857df] SIMDTypes v0.1.0
  [0bca4576] SciMLBase v2.104.0
  [19f34311] SciMLJacobianOperators v0.1.7
  [c0aeaf25] SciMLOperators v1.3.1
  [431bcebd] SciMLPublic v1.0.0
  [53ae85a6] SciMLStructures v1.7.0
  [6c6a2e73] Scratch v1.3.0
  [91c51154] SentinelArrays v1.4.8
  [efcf1570] Setfield v1.1.2
  [727e6d20] SimpleNonlinearSolve v2.6.0
  [699a6c99] SimpleTraits v0.9.4
  [ce78b400] SimpleUnPack v1.1.0
  [a2af1166] SortingAlgorithms v1.2.1
  [0a514795] SparseMatrixColorings v0.4.21
  [276daf66] SpecialFunctions v2.5.1
  [aedffcd0] Static v1.2.0
  [0d7ed370] StaticArrayInterface v1.8.0
  [90137ffa] StaticArrays v1.9.14
  [1e83bf80] StaticArraysCore v1.4.3
  [10745b16] Statistics v1.11.1
  [82ae8749] StatsAPI v1.7.1
  [2913bbd2] StatsBase v0.34.5
  [4c63d2b9] StatsFuns v1.5.0
  [9672c7b4] SteadyStateDiffEq v2.5.0
  [7792a7ef] StrideArraysCore v0.5.7
  [69024149] StringEncodings v0.3.7
  [892a3eda] StringManipulation v0.4.1
  [354b36f9] StringViews v1.3.4
⌅ [09ab397b] StructArrays v0.6.21
  [856f2bd8] StructTypes v1.11.0
  [9c9a347c] SymbolicAWEModels v0.1.3
  [2efcf032] SymbolicIndexingInterface v0.3.42
  [19f23fe9] SymbolicLimits v0.2.2
  [d1185830] SymbolicUtils v3.29.0
  [0c5d862f] Symbolics v6.45.0
  [dc5dba14] TZJData v1.5.0+2025b
  [3783bdb8] TableTraits v1.0.1
  [bd369af6] Tables v1.12.1
  [ed4db957] TaskLocalValues v0.1.3
  [8ea1fca8] TermInterface v2.0.0
  [1e6cf692] TestEnv v1.102.1
  [1c621080] TestItems v1.0.0
  [8290d209] ThreadingUtilities v0.5.5
  [f269a46b] TimeZones v1.22.0
  [a759f4b9] TimerOutputs v0.5.29
  [21f18d07] Timers v0.1.5
  [3bb67fe8] TranscodingStreams v0.11.3
  [410a4b4d] Tricks v0.1.10
  [781d530d] TruncatedStacktraces v1.4.0
  [5c2747f8] URIs v1.6.1
  [3a884ed6] UnPack v1.0.2
  [1986cc42] Unitful v1.23.1
  [a7c27f48] Unityper v0.1.6
  [ed3cd733] VortexStepMethod v2.0.0
  [ea10d353] WeakRefStrings v1.4.2
  [897b6980] WeakValueDicts v0.1.0
  [7dcfa46b] WinchModels v0.3.7
  [efce3f68] WoodburyMatrices v1.0.0
  [76eceee3] WorkerUtilities v1.6.1
⌅ [19641d66] Xfoil v0.5.0
  [ddb6d928] YAML v0.4.14
  [a5390f91] ZipFile v0.10.1
  [f5851436] FFTW_jll v3.3.11+0
  [1d5cc7b8] IntelOpenMP_jll v2025.2.0+0
  [94ce4f54] Libiconv_jll v1.18.0+0
  [5ced341a] Lz4_jll v1.10.1+0
  [856f044c] MKL_jll v2025.2.0+0
  [efe28fd5] OpenSpecFun_jll v0.5.6+0
  [f50d1b31] Rmath_jll v0.5.1+0
  [ffd25f8a] XZ_jll v5.8.1+0
  [3161d3a3] Zstd_jll v1.5.7+1
  [1317d2d5] oneTBB_jll v2022.0.0+0
  [70cc596b] xfoil_light_jll v0.2.1+0
  [0dad84c5] ArgTools v1.1.2
  [56f22d72] Artifacts v1.11.0
  [2a0f44e3] Base64 v1.11.0
  [ade2ca70] Dates v1.11.0
  [8ba89e20] Distributed v1.11.0
  [f43a241f] Downloads v1.6.0
  [7b1f6079] FileWatching v1.11.0
  [9fa8497b] Future v1.11.0
  [b77e0a4c] InteractiveUtils v1.11.0
  [4af54fe1] LazyArtifacts v1.11.0
  [b27032c2] LibCURL v0.6.4
  [76f85450] LibGit2 v1.11.0
  [8f399da3] Libdl v1.11.0
  [37e2e46d] LinearAlgebra v1.11.0
  [56ddb016] Logging v1.11.0
  [d6f4376e] Markdown v1.11.0
  [a63ad114] Mmap v1.11.0
  [ca575930] NetworkOptions v1.2.0
  [44cfe95a] Pkg v1.11.0
  [de0858da] Printf v1.11.0
  [3fa0cd96] REPL v1.11.0
  [9a3f8284] Random v1.11.0
  [ea8e919c] SHA v0.7.0
  [9e88b42a] Serialization v1.11.0
  [1a1011a3] SharedArrays v1.11.0
  [6462fe0b] Sockets v1.11.0
  [2f01184e] SparseArrays v1.11.0
  [f489334b] StyledStrings v1.11.0
  [4607b0f0] SuiteSparse
  [fa267f1f] TOML v1.0.3
  [a4e569a6] Tar v1.10.0
  [8dfed614] Test v1.11.0
  [cf7118a7] UUIDs v1.11.0
  [4ec0a83e] Unicode v1.11.0
  [e66e0078] CompilerSupportLibraries_jll v1.1.1+0
  [deac9b47] LibCURL_jll v8.6.0+0
  [e37daf67] LibGit2_jll v1.7.2+0
  [29816b5a] LibSSH2_jll v1.11.0+1
  [c8ffd9c3] MbedTLS_jll v2.28.6+0
  [14a3606d] MozillaCACerts_jll v2023.12.12
  [4536629a] OpenBLAS_jll v0.3.27+1
  [05823500] OpenLibm_jll v0.8.5+0
  [bea87d4a] SuiteSparse_jll v7.7.0+0
  [83775a58] Zlib_jll v1.2.13+1
  [8e850b90] libblastrampoline_jll v5.11.0+0
  [8e850ede] nghttp2_jll v1.59.0+0
  [3f19e933] p7zip_jll v17.4.0+2

• Output of versioninfo()

Julia Version 1.11.6
Commit 9615af0f269 (2025-07-09 12:58 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 16 × Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, skylake)
Threads: 1 default, 0 interactive, 1 GC (on 16 virtual cores)

[AayushSabharwal]

MTK doesn't handle array equations very well. Depending on what version you updated from, we might have changed how simplification works in a way that causes it to hit this issue.

[AayushSabharwal]

#3003 is essentially the tracker for this issue

[1-Bart-1]

Are there plans to fix this? And on which timescale?

[AayushSabharwal]

There are plans to fix this, but it will take a considerable amount of time. The way to do it is to properly support arrays. This requires (in order):

1. Proper support for array symbolics and array algebra in SymbolicUtils
2. Array calculus in Symbolics
3. Refactoring simplification in MTK to handle array equations

None of these is trivial, and we're still working on the first one as part of SymbolicUtils@4