diff --git a/src/systems/diffeqs/abstractodesystem.jl b/src/systems/diffeqs/abstractodesystem.jl
index 5f69266f7e..f61423e61d 100644
--- a/src/systems/diffeqs/abstractodesystem.jl
+++ b/src/systems/diffeqs/abstractodesystem.jl
@@ -762,7 +762,7 @@ function process_DEProblem(constructor, sys::AbstractODESystem, u0map, parammap;
         warn_initialize_determined = true,
         build_initializeprob = true,
         initialization_eqs = [],
-        fully_determined = false,
+        fully_determined = nothing,
         kwargs...)
     eqs = equations(sys)
     dvs = unknowns(sys)
@@ -1467,7 +1467,7 @@ InitializationProblem{iip}(sys::AbstractODESystem, u0map, tspan,
                            simplify = false,
                            linenumbers = true, parallel = SerialForm(),
                            initialization_eqs = [],
-                           fully_determined = false,
+                           fully_determined = nothing,
                            kwargs...) where {iip}
 ```
 
@@ -1517,7 +1517,7 @@ function InitializationProblem{iip, specialize}(sys::AbstractODESystem,
         check_length = true,
         warn_initialize_determined = true,
         initialization_eqs = [],
-        fully_determined = false,
+        fully_determined = nothing,
         kwargs...) where {iip, specialize}
     if !iscomplete(sys)
         error("A completed system is required. Call `complete` or `structural_simplify` on the system before creating an `ODEProblem`")
@@ -1525,11 +1525,13 @@ function InitializationProblem{iip, specialize}(sys::AbstractODESystem,
     if isempty(u0map) && get_initializesystem(sys) !== nothing
         isys = get_initializesystem(sys; initialization_eqs)
     elseif isempty(u0map) && get_initializesystem(sys) === nothing
-        isys = structural_simplify(
-            generate_initializesystem(sys; initialization_eqs); fully_determined)
+        isys = generate_initializesystem(sys; initialization_eqs)
+        _fully_determined = fully_determined === nothing ? length(equations(isys)) == length(unknowns(isys))
+        isys = structural_simplify(isys ; _fully_determined)
     else
-        isys = structural_simplify(
-            generate_initializesystem(sys; u0map, initialization_eqs); fully_determined)
+        isys = generate_initializesystem(sys; u0map, initialization_eqs)
+        _fully_determined = fully_determined === nothing ? length(equations(isys)) == length(unknowns(isys))
+        isys = structural_simplify(isys; _fully_determined)
     end
 
     uninit = setdiff(unknowns(sys), [unknowns(isys); getfield.(observed(isys), :lhs)])