fix: fix add_fallbacks!

src/systems/problem_utils.jl

@@ -88,6 +88,18 @@ function symbols_to_symbolics!(sys::AbstractSystem, varmap::AbstractDict)
     end
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+Utility function to get the value `val` corresponding to key `var` in `varmap`, and
+return `getindex(val, idx)` if it exists or `nothing` otherwise.
+"""
+function get_and_getindex(varmap, var, idx)
+    val = get(varmap, var, nothing)
+    val === nothing && return nothing
+    return val[idx]
+end
+
 """
     $(TYPEDSIGNATURES)
 
@@ -115,8 +127,9 @@ function add_fallbacks!(
             val = map(eachindex(var)) do idx
                 # @something is lazy and saves from writing a massive if-elseif-else
                 @something(get(varmap, var[idx], nothing),
-                    get(varmap, ttvar[idx], nothing), get(fallbacks, var, nothing)[idx],
-                    get(fallbacks, ttvar, nothing)[idx], get(fallbacks, var[idx], nothing),
+                    get(varmap, ttvar[idx], nothing), get_and_getindex(fallbacks, var, idx),
+                    get_and_getindex(fallbacks, ttvar, idx), get(
+                        fallbacks, var[idx], nothing),
                     get(fallbacks, ttvar[idx], nothing), Some(nothing))
             end
             # only push the missing entries

test/initial_values.jl

@@ -174,3 +174,11 @@ end
     @test_throws ModelingToolkit.UnexpectedSymbolicValueInVarmap ODEProblem(
         sys, [x => 1, y => 2], (0.0, 1.0), [p => 2q, q => 3p])
 end
+
+@testset "`add_fallbacks!` checks scalarized array parameters correctly" begin
+    @variables x(t)[1:2]
+    @parameters p[1:2, 1:2]
+    @mtkbuild sys = ODESystem(D(x) ~ p * x, t)
+    # used to throw a `MethodError` complaining about `getindex(::Nothing, ::CartesianIndex{2})`
+    @test_throws ModelingToolkit.MissingParametersError ODEProblem(sys, [x => ones(2)], (0.0, 1.0))
+end