diff --git a/internal/stacks/stackruntime/internal/stackeval/input_variable.go b/internal/stacks/stackruntime/internal/stackeval/input_variable.go
index 68e0482baf..da9c6a4596 100644
--- a/internal/stacks/stackruntime/internal/stackeval/input_variable.go
+++ b/internal/stacks/stackruntime/internal/stackeval/input_variable.go
@@ -113,9 +113,10 @@ func (v *InputVariable) CheckValue(ctx context.Context, phase EvalPhase) (cty.Va
 
 				extVal := v.main.RootVariableValue(ctx, v.Addr().Item, phase)
 
-				// If the calling context does not define a value for this
-				// variable, we need to fall back to the default.
-				if extVal.Value == cty.NilVal {
+				// We treat a null value as equivalent to an unspecified value,
+				// and replace it with the variable's default value. This is
+				// consistent with how embedded stacks handle defaults.
+				if extVal.Value.IsNull() {
 					cfg := v.Config(ctx)
 
 					// A separate code path will validate the default value, so
diff --git a/internal/stacks/stackruntime/internal/stackeval/main.go b/internal/stacks/stackruntime/internal/stackeval/main.go
index e2772bc76b..bd10ae6ff3 100644
--- a/internal/stacks/stackruntime/internal/stackeval/main.go
+++ b/internal/stacks/stackruntime/internal/stackeval/main.go
@@ -455,11 +455,11 @@ func (m *Main) RootVariableValue(ctx context.Context, addr stackaddrs.InputVaria
 		ret, ok := m.planning.opts.InputVariableValues[addr]
 		if !ok {
 			// If no value is specified for the given input variable, we return
-			// a nil placeholder. Nil can never be specified, so the caller can
-			// determine that the variable's default value should be used (if
-			// present) or an error raised (if not).
+			// a null value. Callers should treat a null value as equivalent to
+			// an unspecified one, applying default (if present) or raising an
+			// error (if not).
 			return ExternalInputValue{
-				Value: cty.NilVal,
+				Value: cty.NullVal(cty.DynamicPseudoType),
 			}
 		}
 		return ret
diff --git a/internal/stacks/stackruntime/plan_test.go b/internal/stacks/stackruntime/plan_test.go
index 7fd9f5f5d8..b7910e6897 100644
--- a/internal/stacks/stackruntime/plan_test.go
+++ b/internal/stacks/stackruntime/plan_test.go
@@ -285,6 +285,91 @@ func TestPlanWithNoValueForRequiredVariable(t *testing.T) {
 	}
 }
 
+func TestPlanWithVariableDefaults(t *testing.T) {
+	// Test that defaults are applied correctly for both unspecified input
+	// variables and those with an explicit null value.
+	testCases := map[string]struct {
+		inputs map[stackaddrs.InputVariable]ExternalInputValue
+	}{
+		"unspecified": {
+			inputs: make(map[stackaddrs.InputVariable]ExternalInputValue),
+		},
+		"explicit null": {
+			inputs: map[stackaddrs.InputVariable]ExternalInputValue{
+				stackaddrs.InputVariable{Name: "beep"}: ExternalInputValue{
+					Value:    cty.NullVal(cty.DynamicPseudoType),
+					DefRange: tfdiags.SourceRange{Filename: "fake.tfstack.hcl"},
+				},
+			},
+		},
+	}
+
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			ctx := context.Background()
+			cfg := loadMainBundleConfigForTest(t, "plan-variable-defaults")
+
+			changesCh := make(chan stackplan.PlannedChange, 8)
+			diagsCh := make(chan tfdiags.Diagnostic, 2)
+			req := PlanRequest{
+				Config:      cfg,
+				InputValues: tc.inputs,
+			}
+			resp := PlanResponse{
+				PlannedChanges: changesCh,
+				Diagnostics:    diagsCh,
+			}
+
+			go Plan(ctx, &req, &resp)
+			gotChanges, diags := collectPlanOutput(changesCh, diagsCh)
+
+			if len(diags) != 0 {
+				t.Errorf("unexpected diagnostics\n%s", diags.ErrWithWarnings().Error())
+			}
+
+			wantChanges := []stackplan.PlannedChange{
+				&stackplan.PlannedChangeApplyable{
+					Applyable: true,
+				},
+				&stackplan.PlannedChangeHeader{
+					TerraformVersion: version.SemVer,
+				},
+				&stackplan.PlannedChangeOutputValue{
+					Addr:     stackaddrs.OutputValue{Name: "beep"},
+					Action:   plans.Create,
+					OldValue: plans.DynamicValue{0xc0},               // MessagePack nil
+					NewValue: plans.DynamicValue([]byte("\xa4BEEP")), // MessagePack string "BEEP"
+				},
+				&stackplan.PlannedChangeOutputValue{
+					Addr:     stackaddrs.OutputValue{Name: "defaulted"},
+					Action:   plans.Create,
+					OldValue: plans.DynamicValue{0xc0},               // MessagePack nil
+					NewValue: plans.DynamicValue([]byte("\xa4BOOP")), // MessagePack string "BOOP"
+				},
+				&stackplan.PlannedChangeOutputValue{
+					Addr:     stackaddrs.OutputValue{Name: "specified"},
+					Action:   plans.Create,
+					OldValue: plans.DynamicValue{0xc0},               // MessagePack nil
+					NewValue: plans.DynamicValue([]byte("\xa4BEEP")), // MessagePack string "BEEP"
+				},
+				&stackplan.PlannedChangeRootInputValue{
+					Addr: stackaddrs.InputVariable{
+						Name: "beep",
+					},
+					Value: cty.StringVal("BEEP"),
+				},
+			}
+			sort.SliceStable(gotChanges, func(i, j int) bool {
+				return plannedChangeSortKey(gotChanges[i]) < plannedChangeSortKey(gotChanges[j])
+			})
+
+			if diff := cmp.Diff(wantChanges, gotChanges, ctydebug.CmpOptions); diff != "" {
+				t.Errorf("wrong changes\n%s", diff)
+			}
+		})
+	}
+}
+
 func TestPlanWithSingleResource(t *testing.T) {
 	ctx := context.Background()
 	cfg := loadMainBundleConfigForTest(t, "with-single-resource")
diff --git a/internal/stacks/stackruntime/testdata/mainbundle/test/plan-variable-defaults/child/child.tfstack.hcl b/internal/stacks/stackruntime/testdata/mainbundle/test/plan-variable-defaults/child/child.tfstack.hcl
new file mode 100644
index 0000000000..7fa976ffda
--- /dev/null
+++ b/internal/stacks/stackruntime/testdata/mainbundle/test/plan-variable-defaults/child/child.tfstack.hcl
@@ -0,0 +1,9 @@
+variable "boop" {
+  type    = string
+  default = "BOOP"
+}
+
+output "result" {
+  type  = string
+  value = var.boop
+}
\ No newline at end of file
diff --git a/internal/stacks/stackruntime/testdata/mainbundle/test/plan-variable-defaults/deployments.tfdeploy.hcl b/internal/stacks/stackruntime/testdata/mainbundle/test/plan-variable-defaults/deployments.tfdeploy.hcl
new file mode 100644
index 0000000000..811bbfdf56
--- /dev/null
+++ b/internal/stacks/stackruntime/testdata/mainbundle/test/plan-variable-defaults/deployments.tfdeploy.hcl
@@ -0,0 +1,3 @@
+deployment "main" {
+  inputs = {}
+}
diff --git a/internal/stacks/stackruntime/testdata/mainbundle/test/plan-variable-defaults/plan-variable-default.tfstack.hcl b/internal/stacks/stackruntime/testdata/mainbundle/test/plan-variable-defaults/plan-variable-default.tfstack.hcl
new file mode 100644
index 0000000000..37d763eeef
--- /dev/null
+++ b/internal/stacks/stackruntime/testdata/mainbundle/test/plan-variable-defaults/plan-variable-default.tfstack.hcl
@@ -0,0 +1,31 @@
+variable "beep" {
+  type    = string
+  default = "BEEP"
+}
+
+output "beep" {
+  type  = string
+  value = var.beep
+}
+
+stack "specified" {
+  source = "./child"
+  inputs = {
+    boop = var.beep
+  }
+}
+
+stack "defaulted" {
+  source = "./child"
+  inputs = {}
+}
+
+output "specified" {
+  type = string
+  value = stack.specified.result
+}
+
+output "defaulted" {
+  type = string
+  value = stack.defaulted.result
+}
diff --git a/internal/stacks/stackruntime/validate_test.go b/internal/stacks/stackruntime/validate_test.go
index 64cd295dc2..63564a6770 100644
--- a/internal/stacks/stackruntime/validate_test.go
+++ b/internal/stacks/stackruntime/validate_test.go
@@ -38,6 +38,7 @@ var (
 	// validConfigurations are shared between the validate and plan tests.
 	validConfigurations = map[string]validateTestInput{
 		"empty":                            {},
+		"plan-variable-defaults":           {},
 		"variable-output-roundtrip":        {},
 		"variable-output-roundtrip-nested": {},
 		filepath.Join("with-single-input", "input-from-component"): {},