diff --git a/pkg/blueprint/disk_customizations.go b/pkg/blueprint/disk_customizations.go
index 40ecfdab31..0fd360bfb7 100644
--- a/pkg/blueprint/disk_customizations.go
+++ b/pkg/blueprint/disk_customizations.go
@@ -113,13 +113,15 @@ func (lv *LVCustomization) UnmarshalJSON(data []byte) error {
 	lv.Name = lvAnySize.Name
 	lv.FilesystemTypedCustomization = lvAnySize.FilesystemTypedCustomization
 
-	if lvAnySize.MinSize != nil {
-		size, err := decodeSize(lvAnySize.MinSize)
-		if err != nil {
-			return err
-		}
-		lv.MinSize = size
+	if lvAnySize.MinSize == nil {
+		return fmt.Errorf("minsize is required")
+	}
+	size, err := decodeSize(lvAnySize.MinSize)
+	if err != nil {
+		return err
 	}
+	lv.MinSize = size
+
 	return nil
 }
 
@@ -177,14 +179,16 @@ func (v *PartitionCustomization) UnmarshalJSON(data []byte) error {
 
 	v.Type = partType
 
-	if typeSniffer.MinSize != nil {
-		minsize, err := decodeSize(typeSniffer.MinSize)
-		if err != nil {
-			return fmt.Errorf("%s error decoding minsize for partition: %w", errPrefix, err)
-		}
-		v.MinSize = minsize
+	if typeSniffer.MinSize == nil {
+		return fmt.Errorf("minsize is required")
 	}
 
+	minsize, err := decodeSize(typeSniffer.MinSize)
+	if err != nil {
+		return fmt.Errorf("%s error decoding minsize for partition: %w", errPrefix, err)
+	}
+	v.MinSize = minsize
+
 	return nil
 }
 
@@ -301,13 +305,15 @@ func (v *PartitionCustomization) UnmarshalTOML(data any) error {
 
 	v.Type = partType
 
-	if minsizeField, ok := d["minsize"]; ok {
-		minsize, err := decodeSize(minsizeField)
-		if err != nil {
-			return fmt.Errorf("%s error decoding minsize for partition: %w", errPrefix, err)
-		}
-		v.MinSize = minsize
+	minsizeField, ok := d["minsize"]
+	if !ok {
+		return fmt.Errorf("minsize is required")
+	}
+	minsize, err := decodeSize(minsizeField)
+	if err != nil {
+		return fmt.Errorf("%s error decoding minsize for partition: %w", errPrefix, err)
 	}
+	v.MinSize = minsize
 
 	return nil
 }
diff --git a/pkg/blueprint/disk_customizations_test.go b/pkg/blueprint/disk_customizations_test.go
index 7a22d8b779..9c3259b5dc 100644
--- a/pkg/blueprint/disk_customizations_test.go
+++ b/pkg/blueprint/disk_customizations_test.go
@@ -1035,16 +1035,8 @@ func TestPartitionCustomizationUnmarshalJSON(t *testing.T) {
 
 	testCases := map[string]testCase{
 		"nothing": {
-			input: "{}",
-			expected: &blueprint.PartitionCustomization{
-				Type:    "plain",
-				MinSize: 0,
-				FilesystemTypedCustomization: blueprint.FilesystemTypedCustomization{
-					Mountpoint: "",
-					Label:      "",
-					FSType:     "",
-				},
-			},
+			input:    "{}",
+			errorMsg: "minsize is required",
 		},
 		"plain": {
 			input: `{
@@ -1346,16 +1338,8 @@ func TestPartitionCustomizationUnmarshalTOML(t *testing.T) {
 
 	testCases := map[string]testCase{
 		"nothing": {
-			input: "",
-			expected: &blueprint.PartitionCustomization{
-				Type:    "plain",
-				MinSize: 0,
-				FilesystemTypedCustomization: blueprint.FilesystemTypedCustomization{
-					Mountpoint: "",
-					Label:      "",
-					FSType:     "",
-				},
-			},
+			input:    "",
+			errorMsg: "toml: line 0: minsize is required",
 		},
 		"plain": {
 			input: `type = "plain"
@@ -1647,13 +1631,6 @@ func TestDiskCustomizationUnmarshalJSON(t *testing.T) {
 	}
 
 	testCases := map[string]testCase{
-		"nothing": {
-			inputJSON: "{}",
-			inputTOML: "",
-			expected: &blueprint.DiskCustomization{
-				MinSize: 0,
-			},
-		},
 		"minsize/int": {
 			inputJSON: `{
 				"minsize": 1234