diff --git a/pkg/blueprint/filesystem_customizations.go b/pkg/blueprint/filesystem_customizations.go
index e783930052..7f9d1c1918 100644
--- a/pkg/blueprint/filesystem_customizations.go
+++ b/pkg/blueprint/filesystem_customizations.go
@@ -10,49 +10,43 @@ import (
 )
 
 type FilesystemCustomization struct {
-	Mountpoint string `json:"mountpoint,omitempty" toml:"mountpoint,omitempty"`
-	MinSize    uint64 `json:"minsize,omitempty" toml:"minsize,omitempty"`
+	Mountpoint string
+	MinSize    uint64
 }
 
-func (fsc *FilesystemCustomization) UnmarshalTOML(data interface{}) error {
-	d, ok := data.(map[string]interface{})
-	if !ok {
-		return fmt.Errorf("customizations.filesystem is not an object")
-	}
+type filesystemCustomizationMarshaling struct {
+	Mountpoint string         `json:"mountpoint,omitempty" toml:"mountpoint,omitempty"`
+	MinSize    datasizes.Size `json:"minsize,omitempty" toml:"minsize,omitempty"`
+}
 
-	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"])
-	}
-	minSize, err := decodeSize(d["minsize"])
+func (fsc *FilesystemCustomization) UnmarshalTOML(data any) error {
+	// This is the most efficient way to reuse code when unmarshaling
+	// structs in toml, it leaks json errors which is a bit sad but
+	// because the toml unmarshaler gives us not "[]byte" but an
+	// already pre-processed "any" we cannot just unmarshal into our
+	// "fooMarshaling" struct and reuse the result so we resort to
+	// this workaround (but toml will go away long term anyway).
+	dataJSON, err := json.Marshal(data)
 	if err != nil {
-		return fmt.Errorf("TOML unmarshal: error decoding minsize value for mountpoint %q: %w", fsc.Mountpoint, err)
+		return fmt.Errorf("error unmarshaling TOML data %v: %w", data, err)
+	}
+	if err := fsc.UnmarshalJSON(dataJSON); err != nil {
+		return fmt.Errorf("error decoding TOML %v: %w", data, err)
 	}
-	fsc.MinSize = minSize
 	return nil
 }
 
 func (fsc *FilesystemCustomization) UnmarshalJSON(data []byte) error {
-	var v interface{}
-	if err := json.Unmarshal(data, &v); err != nil {
+	var fc filesystemCustomizationMarshaling
+	if err := json.Unmarshal(data, &fc); err != nil {
+		if fc.Mountpoint != "" {
+			return fmt.Errorf("error decoding minsize value for mountpoint %q: %w", fc.Mountpoint, err)
+		}
 		return err
 	}
-	d, _ := v.(map[string]interface{})
+	fsc.Mountpoint = fc.Mountpoint
+	fsc.MinSize = fc.MinSize.Uint64()
 
-	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"])
-	}
-
-	minSize, err := decodeSize(d["minsize"])
-	if err != nil {
-		return fmt.Errorf("JSON unmarshal: error decoding minsize value for mountpoint %q: %w", fsc.Mountpoint, err)
-	}
-	fsc.MinSize = minSize
 	return nil
 }
 
diff --git a/pkg/blueprint/filesystem_customizations_test.go b/pkg/blueprint/filesystem_customizations_test.go
index fbb85f0817..0c12f6cb68 100644
--- a/pkg/blueprint/filesystem_customizations_test.go
+++ b/pkg/blueprint/filesystem_customizations_test.go
@@ -47,19 +47,19 @@ func TestFilesystemCustomizationUnmarshalTOMLUnhappy(t *testing.T) {
 			name: "mountpoint not string",
 			input: `mountpoint = 42
 			minsize = 42`,
-			err: `toml: line 0: TOML unmarshal: mountpoint must be string, got "42" of type int64`,
+			err: `toml: line 0: error decoding TOML map[minsize:42 mountpoint:42]: json: cannot unmarshal number into Go struct field filesystemCustomizationMarshaling.mountpoint of type string`,
 		},
 		{
 			name: "minsize nor string nor int",
 			input: `mountpoint="/"
 			minsize = true`,
-			err: `toml: line 0: TOML unmarshal: error decoding minsize value for mountpoint "/": failed to convert value "true" to number`,
+			err: `toml: line 0: error decoding TOML map[minsize:true mountpoint:/]: error decoding size: failed to convert value "true" to number`,
 		},
 		{
 			name: "minsize not parseable",
 			input: `mountpoint="/"
 			minsize = "20 KG"`,
-			err: `toml: line 0: TOML unmarshal: error decoding minsize value for mountpoint "/": unknown data size units in string: 20 KG`,
+			err: `toml: line 0: error decoding TOML map[minsize:20 KG mountpoint:/]: error decoding size: unknown data size units in string: 20 KG`,
 		},
 	}
 
@@ -81,17 +81,17 @@ func TestFilesystemCustomizationUnmarshalJSONUnhappy(t *testing.T) {
 		{
 			name:  "mountpoint not string",
 			input: `{"mountpoint": 42, "minsize": 42}`,
-			err:   `JSON unmarshal: mountpoint must be string, got "42" of type float64`,
+			err:   `json: cannot unmarshal number into Go struct field filesystemCustomizationMarshaling.mountpoint of type string`,
 		},
 		{
 			name:  "minsize nor string nor int",
 			input: `{"mountpoint":"/", "minsize": true}`,
-			err:   `JSON unmarshal: error decoding minsize value for mountpoint "/": failed to convert value "true" to number`,
+			err:   `error decoding minsize value for mountpoint "/": error decoding size: failed to convert value "true" to number`,
 		},
 		{
 			name:  "minsize not parseable",
 			input: `{ "mountpoint": "/", "minsize": "20 KG"}`,
-			err:   `JSON unmarshal: error decoding minsize value for mountpoint "/": unknown data size units in string: 20 KG`,
+			err:   `error decoding minsize value for mountpoint "/": error decoding size: unknown data size units in string: 20 KG`,
 		},
 	}
 
@@ -115,7 +115,7 @@ func TestFilesystemCustomizationUnmarshalTOMLNotAnObject(t *testing.T) {
 			input: `
 [customizations]
 filesystem = ["hello"]`,
-			err: "toml: line 3 (last key \"customizations.filesystem\"): customizations.filesystem is not an object",
+			err: `toml: line 3 (last key "customizations.filesystem"): error decoding TOML hello: json: cannot unmarshal string into Go value of type blueprint.filesystemCustomizationMarshaling`,
 		},
 	}