diff --git a/base/compiler/ssair/passes.jl b/base/compiler/ssair/passes.jl
index 114b8c6033615..40d0a14068a29 100644
--- a/base/compiler/ssair/passes.jl
+++ b/base/compiler/ssair/passes.jl
@@ -571,7 +571,7 @@ function getfield_elim_pass!(ir::IRCode)
         elseif is_known_call(stmt, isa, compact)
             # TODO
             continue
-        elseif is_known_call(stmt, typeassert, compact)
+        elseif is_known_call(stmt, typeassert, compact) && length(stmt.args) == 3
             # Canonicalize
             #   X = typeassert(Y, T)::S
             # into
@@ -592,7 +592,7 @@ function getfield_elim_pass!(ir::IRCode)
                     compact.result[idx][:line]), true)
             compact.ssa_rename[compact.idx-1] = pi
             continue
-        elseif is_known_call(stmt, (===), compact)
+        elseif is_known_call(stmt, (===), compact) && length(stmt.args) == 3
             c1 = compact_exprtype(compact, stmt.args[2])
             c2 = compact_exprtype(compact, stmt.args[3])
             if !(isa(c1, Const) || isa(c2, Const))
diff --git a/test/compiler/inference.jl b/test/compiler/inference.jl
index 5cbd39d8468a6..2c49042892629 100644
--- a/test/compiler/inference.jl
+++ b/test/compiler/inference.jl
@@ -3313,3 +3313,7 @@ end |> first === Tuple{Int, String}
 
 # issue #40742
 @test Base.return_types(string, (Vector{Tuple{:x}},)) == Any[String]
+
+# issue #40804
+@test Base.return_types(()) do; ===(); end == Any[Union{}]
+@test Base.return_types(()) do; typeassert(); end == Any[Union{}]