diff --git a/internal/blueprint/customizations.go b/internal/blueprint/customizations.go
index 3b95983b9..a0768c2dc 100644
--- a/internal/blueprint/customizations.go
+++ b/internal/blueprint/customizations.go
@@ -1,6 +1,7 @@
 package blueprint
 
 import (
+	"encoding/json"
 	"fmt"
 	"reflect"
 )
@@ -76,6 +77,52 @@ type FilesystemCustomization struct {
 	MinSize    uint64 `json:"minsize,omitempty" toml:"size,omitempty"`
 }
 
+func (fsc *FilesystemCustomization) UnmarshalTOML(data interface{}) error {
+	d, _ := data.(map[string]interface{})
+
+	switch d["mountpoint"].(type) {
+	case string:
+		fsc.Mountpoint = d["mountpoint"].(string)
+	default:
+		return fmt.Errorf("TOML unmarshal: mountpoint must be string, got %v of type %T", d["mountpoint"], d["mountpoint"])
+	}
+
+	switch d["size"].(type) {
+	case int64:
+		fsc.MinSize = uint64(d["size"].(int64))
+	default:
+		return fmt.Errorf("TOML unmarshal: size must be integer, got %v of type %T", d["size"], d["size"])
+	}
+
+	return nil
+}
+
+func (fsc *FilesystemCustomization) UnmarshalJSON(data []byte) error {
+	var v interface{}
+	if err := json.Unmarshal(data, &v); err != nil {
+		return err
+	}
+	d, _ := v.(map[string]interface{})
+
+	switch d["mountpoint"].(type) {
+	case string:
+		fsc.Mountpoint = d["mountpoint"].(string)
+	default:
+		return fmt.Errorf("JSON unmarshal: mountpoint must be string, got %v of type %T", d["mountpoint"], d["mountpoint"])
+	}
+
+	// The JSON specification only mentions float64 and Go defaults to it: https://go.dev/blog/json
+	switch d["minsize"].(type) {
+	case float64:
+		// Note that it uses different key than the TOML version
+		fsc.MinSize = uint64(d["minsize"].(float64))
+	default:
+		return fmt.Errorf("JSON unmarshal: minsize must be float64 number, got %v of type %T", d["minsize"], d["minsize"])
+	}
+
+	return nil
+}
+
 type CustomizationError struct {
 	Message string
 }