diff --git a/src/systems/diffeqs/abstractodesystem.jl b/src/systems/diffeqs/abstractodesystem.jl
index ab0565eb31..343d2856ca 100644
--- a/src/systems/diffeqs/abstractodesystem.jl
+++ b/src/systems/diffeqs/abstractodesystem.jl
@@ -1562,14 +1562,17 @@ function InitializationProblem{iip, specialize}(sys::AbstractODESystem,
     parammap = parammap isa DiffEqBase.NullParameters || isempty(parammap) ?
                [get_iv(sys) => t] :
                merge(todict(parammap), Dict(get_iv(sys) => t))
-    if isempty(u0map)
-        u0map = Dict()
-    end
-    if isempty(guesses)
-        guesses = Dict()
-    end
+    u0map = isempty(u0map) ? Dict() : todict(u0map)
+    guesses = isempty(guesses) ? Dict() : todict(guesses)
+    defs = defaults(isys)
+
+    # Check that all unknowns have guesses
+    unknowns_given = union(keys(guesses), keys(defs), keys(u0map))
+    unknowns_missing = setdiff(unknowns(isys), unknowns_given)
+    !isempty(unknowns_missing) &&
+        throw(ArgumentError("Missing guesses for unknowns $(join(unknowns_missing, ", "))."))
 
-    u0map = merge(todict(guesses), todict(u0map))
+    u0map = merge(guesses, u0map)
     if neqs == nunknown
         NonlinearProblem(isys, u0map, parammap; kwargs...)
     else
diff --git a/test/initializationsystem.jl b/test/initializationsystem.jl
index f4097ecd97..3cd30dd7e6 100644
--- a/test/initializationsystem.jl
+++ b/test/initializationsystem.jl
@@ -456,3 +456,11 @@ sys = structural_simplify(unsimp; fully_determined = false)
 sys = extend(sysx, sysy)
 @test length(equations(generate_initializesystem(sys))) == 2
 @test length(ModelingToolkit.guesses(sys)) == 1
+
+# https://github.com/SciML/ModelingToolkit.jl/issues/2873
+@testset "Error on missing defaults" begin
+    @variables x(t) y(t)
+    @named sys = ODESystem([x^2 + y^2 ~ 25, D(x) ~ 1], t)
+    ssys = structural_simplify(sys)
+    @test_throws ArgumentError ODEProblem(ssys, [x => 3], (0, 1), []) # y should have a guess
+end