diff --git a/backend/local/backend_plan.go b/backend/local/backend_plan.go
index 6d2639d323..a50426f3a1 100644
--- a/backend/local/backend_plan.go
+++ b/backend/local/backend_plan.go
@@ -114,12 +114,14 @@ func (b *Local) opPlan(
 
 	// Save the plan to disk
 	if path := op.PlanOutPath; path != "" {
-		if op.PlanOutBackend != nil {
-			plan.Backend = *op.PlanOutBackend
-		} else {
-			op.PlanOutBackend = &plans.Backend{}
-			plan.Backend = *op.PlanOutBackend
+		if op.PlanOutBackend == nil {
+			// This is always a bug in the operation caller; it's not valid
+			// to set PlanOutPath without also setting PlanOutBackend.
+			diags = diags.Append(fmt.Errorf("PlanOutPath set without also setting PlanOutBackend (this is a bug in Terraform)"))
+			b.ReportResult(runningOp, diags)
+			return
 		}
+		plan.Backend = *op.PlanOutBackend
 
 		// We may have updated the state in the refresh step above, but we
 		// will freeze that updated state in the plan file for now and
diff --git a/backend/local/backend_plan_test.go b/backend/local/backend_plan_test.go
index 7ce9e84521..541a58ad8a 100644
--- a/backend/local/backend_plan_test.go
+++ b/backend/local/backend_plan_test.go
@@ -174,6 +174,18 @@ func TestLocal_planDestroy(t *testing.T) {
 	op.Destroy = true
 	op.PlanRefresh = true
 	op.PlanOutPath = planPath
+	cfg := cty.ObjectVal(map[string]cty.Value{
+		"path": cty.StringVal(b.StatePath),
+	})
+	cfgRaw, err := plans.NewDynamicValue(cfg, cfg.Type())
+	if err != nil {
+		t.Fatal(err)
+	}
+	op.PlanOutBackend = &plans.Backend{
+		// Just a placeholder so that we can generate a valid plan file.
+		Type: "local",
+		Config: cfgRaw,
+	}
 
 	run, err := b.Operation(context.Background(), op)
 	if err != nil {
@@ -213,6 +225,18 @@ func TestLocal_planOutPathNoChange(t *testing.T) {
 	op, configCleanup := testOperationPlan(t, "./test-fixtures/plan")
 	defer configCleanup()
 	op.PlanOutPath = planPath
+	cfg := cty.ObjectVal(map[string]cty.Value{
+		"path": cty.StringVal(b.StatePath),
+	})
+	cfgRaw, err := plans.NewDynamicValue(cfg, cfg.Type())
+	if err != nil {
+		t.Fatal(err)
+	}
+	op.PlanOutBackend = &plans.Backend{
+		// Just a placeholder so that we can generate a valid plan file.
+		Type: "local",
+		Config: cfgRaw,
+	}
 
 	run, err := b.Operation(context.Background(), op)
 	if err != nil {
@@ -314,6 +338,8 @@ func testPlanState() *states.State {
 }
 
 func testReadPlan(t *testing.T, path string) *plans.Plan {
+	t.Helper()
+
 	p, err := planfile.Open(path)
 	if err != nil {
 		t.Fatalf("err: %s", err)
diff --git a/backend/local/testing.go b/backend/local/testing.go
index 8b4710c22a..9266c9ccbe 100644
--- a/backend/local/testing.go
+++ b/backend/local/testing.go
@@ -34,6 +34,10 @@ func TestLocal(t *testing.T) (*Local, func()) {
 			var diags tfdiags.Diagnostics
 			diags = diags.Append(vals...)
 			for _, diag := range diags {
+				// NOTE: Since the caller here is not directly the TestLocal
+				// function, t.Helper doesn't apply and so the log source
+				// isn't correctly shown in the test log output. This seems
+				// unavoidable as long as this is happening so indirectly.
 				t.Log(diag.Description().Summary)
 				if local.CLI != nil {
 					local.CLI.Error(diag.Description().Summary)
diff --git a/plans/planfile/tfplan.go b/plans/planfile/tfplan.go
index 2db909b9fb..e1deeb083a 100644
--- a/plans/planfile/tfplan.go
+++ b/plans/planfile/tfplan.go
@@ -351,6 +351,13 @@ func writeTfplan(plan *plans.Plan, w io.Writer) error {
 		rawPlan.Variables[name] = valueToTfplan(val)
 	}
 
+	if plan.Backend.Type == "" || plan.Backend.Config == nil {
+		// This suggests a bug in the code that created the plan, since it
+		// ought to always have a backend populated, even if it's the default
+		// "local" backend with a local state file.
+		return fmt.Errorf("plan does not have a backend configuration")
+	}
+
 	rawPlan.Backend = &planproto.Backend{
 		Type:      plan.Backend.Type,
 		Config:    valueToTfplan(plan.Backend.Config),