diff --git a/src/systems/diffeqs/odesystem.jl b/src/systems/diffeqs/odesystem.jl
index baa82d9b6f..5d1bae95ec 100644
--- a/src/systems/diffeqs/odesystem.jl
+++ b/src/systems/diffeqs/odesystem.jl
@@ -238,7 +238,8 @@ function ODESystem(deqs::AbstractVector{<:Equation}, iv, dvs, ps;
             :ODESystem, force = true)
     end
     defaults = todict(defaults)
-    defaults = Dict{Any, Any}(value(k) => value(v) for (k, v) in pairs(defaults))
+    defaults = Dict{Any, Any}(value(k) => value(v)
+    for (k, v) in pairs(defaults) if value(v) !== nothing)
     var_to_name = Dict()
     process_variables!(var_to_name, defaults, dvs′)
     process_variables!(var_to_name, defaults, ps′)
diff --git a/src/systems/diffeqs/sdesystem.jl b/src/systems/diffeqs/sdesystem.jl
index 489a2e75db..b1f81240f3 100644
--- a/src/systems/diffeqs/sdesystem.jl
+++ b/src/systems/diffeqs/sdesystem.jl
@@ -203,7 +203,8 @@ function SDESystem(deqs::AbstractVector{<:Equation}, neqs::AbstractArray, iv, dv
             :SDESystem, force = true)
     end
     defaults = todict(defaults)
-    defaults = Dict(value(k) => value(v) for (k, v) in pairs(defaults))
+    defaults = Dict(value(k) => value(v)
+    for (k, v) in pairs(defaults) if value(v) !== nothing)
 
     var_to_name = Dict()
     process_variables!(var_to_name, defaults, dvs′)
diff --git a/src/systems/discrete_system/discrete_system.jl b/src/systems/discrete_system/discrete_system.jl
index 79c48af134..86d23acea0 100644
--- a/src/systems/discrete_system/discrete_system.jl
+++ b/src/systems/discrete_system/discrete_system.jl
@@ -149,7 +149,8 @@ function DiscreteSystem(eqs::AbstractVector{<:Equation}, iv, dvs, ps;
             :DiscreteSystem, force = true)
     end
     defaults = todict(defaults)
-    defaults = Dict(value(k) => value(v) for (k, v) in pairs(defaults))
+    defaults = Dict(value(k) => value(v)
+    for (k, v) in pairs(defaults) if value(v) !== nothing)
 
     var_to_name = Dict()
     process_variables!(var_to_name, defaults, dvs′)
diff --git a/src/systems/jumps/jumpsystem.jl b/src/systems/jumps/jumpsystem.jl
index cdd5f1b2f8..536252fec4 100644
--- a/src/systems/jumps/jumpsystem.jl
+++ b/src/systems/jumps/jumpsystem.jl
@@ -176,7 +176,8 @@ function JumpSystem(eqs, iv, unknowns, ps;
             :JumpSystem, force = true)
     end
     defaults = todict(defaults)
-    defaults = Dict(value(k) => value(v) for (k, v) in pairs(defaults))
+    defaults = Dict(value(k) => value(v)
+    for (k, v) in pairs(defaults) if value(v) !== nothing)
 
     unknowns, ps = value.(unknowns), value.(ps)
     var_to_name = Dict()
diff --git a/src/systems/nonlinear/nonlinearsystem.jl b/src/systems/nonlinear/nonlinearsystem.jl
index 43d4bc8cc5..f854b28737 100644
--- a/src/systems/nonlinear/nonlinearsystem.jl
+++ b/src/systems/nonlinear/nonlinearsystem.jl
@@ -146,7 +146,8 @@ function NonlinearSystem(eqs, unknowns, ps;
     end
     jac = RefValue{Any}(EMPTY_JAC)
     defaults = todict(defaults)
-    defaults = Dict{Any, Any}(value(k) => value(v) for (k, v) in pairs(defaults))
+    defaults = Dict{Any, Any}(value(k) => value(v)
+    for (k, v) in pairs(defaults) if value(v) !== nothing)
 
     unknowns, ps = value.(unknowns), value.(ps)
     var_to_name = Dict()
diff --git a/src/systems/optimization/constraints_system.jl b/src/systems/optimization/constraints_system.jl
index 7fde00e2f4..afb5416aa5 100644
--- a/src/systems/optimization/constraints_system.jl
+++ b/src/systems/optimization/constraints_system.jl
@@ -133,7 +133,8 @@ function ConstraintsSystem(constraints, unknowns, ps;
 
     jac = RefValue{Any}(EMPTY_JAC)
     defaults = todict(defaults)
-    defaults = Dict(value(k) => value(v) for (k, v) in pairs(defaults))
+    defaults = Dict(value(k) => value(v)
+    for (k, v) in pairs(defaults) if value(v) !== nothing)
 
     var_to_name = Dict()
     process_variables!(var_to_name, defaults, unknowns′)
diff --git a/src/systems/optimization/optimizationsystem.jl b/src/systems/optimization/optimizationsystem.jl
index 3a9f8aef74..21e82f15cc 100644
--- a/src/systems/optimization/optimizationsystem.jl
+++ b/src/systems/optimization/optimizationsystem.jl
@@ -111,7 +111,8 @@ function OptimizationSystem(op, unknowns, ps;
         throw(ArgumentError("System names must be unique."))
     end
     defaults = todict(defaults)
-    defaults = Dict(value(k) => value(v) for (k, v) in pairs(defaults))
+    defaults = Dict(value(k) => value(v)
+    for (k, v) in pairs(defaults) if value(v) !== nothing)
 
     var_to_name = Dict()
     process_variables!(var_to_name, defaults, unknowns′)
diff --git a/src/utils.jl b/src/utils.jl
index 86a28dcae6..5d6af76b77 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -252,7 +252,9 @@ end
 
 function collect_defaults!(defs, vars)
     for v in vars
-        (haskey(defs, v) || !hasdefault(unwrap(v))) && continue
+        if haskey(defs, v) || !hasdefault(unwrap(v)) || (def = getdefault(v)) === nothing
+            continue
+        end
         defs[v] = getdefault(v)
     end
     return defs
diff --git a/test/initial_values.jl b/test/initial_values.jl
index 3a7b48901d..4c2acd0aac 100644
--- a/test/initial_values.jl
+++ b/test/initial_values.jl
@@ -91,3 +91,20 @@ eqs = [D(D(z)) ~ ones(2, 2)]
 prob = ODEProblem(sys, [], (0.0, 1.0), [A1 => 0.3])
 @test prob.ps[B1] == 0.3
 @test prob.ps[B2] == 0.7
+
+@testset "default=nothing is skipped" begin
+    @parameters p = nothing
+    @variables x(t)=nothing y(t)
+    for sys in [
+        ODESystem(Equation[], t, [x, y], [p]; defaults = [y => nothing], name = :osys),
+        SDESystem(Equation[], [], t, [x, y], [p]; defaults = [y => nothing], name = :ssys),
+        JumpSystem(Equation[], t, [x, y], [p]; defaults = [y => nothing], name = :jsys),
+        NonlinearSystem(Equation[], [x, y], [p]; defaults = [y => nothing], name = :nsys),
+        OptimizationSystem(
+            Equation[], [x, y], [p]; defaults = [y => nothing], name = :optsys),
+        ConstraintsSystem(
+            Equation[], [x, y], [p]; defaults = [y => nothing], name = :conssys)
+    ]
+        @test isempty(ModelingToolkit.defaults(sys))
+    end
+end