diff --git a/internals/plan/extensions_test.go b/internals/plan/extensions_test.go
index 1c02132fe..bf7cbd54b 100644
--- a/internals/plan/extensions_test.go
+++ b/internals/plan/extensions_test.go
@@ -42,16 +42,15 @@ type planResult struct {
 }
 
 var extensionTests = []struct {
-	extensions             map[string]plan.LayerSectionExtension
-	layers                 []*inputLayer
-	errorString            string
-	combinedPlanResult     *planResult
-	combinedPlanResultYaml string
+	extensions map[string]plan.LayerSectionExtension
+	layers     []*inputLayer
+	error      string
+	result     *planResult
+	resultYaml string
 }{
 	// Index 0: No Sections
 	{
-		combinedPlanResultYaml: string(reindent(`
-			{}`)),
+		resultYaml: "{}\n",
 	},
 	// Index 1: Sections with empty YAML
 	{
@@ -59,12 +58,11 @@ var extensionTests = []struct {
 			"x-field": &xExtension{},
 			"y-field": &yExtension{},
 		},
-		combinedPlanResult: &planResult{
+		result: &planResult{
 			x: &xSection{},
 			y: &ySection{},
 		},
-		combinedPlanResultYaml: string(reindent(`
-			{}`)),
+		resultYaml: "{}\n",
 	},
 	// Index 2: Load file layers invalid section
 	{
@@ -73,7 +71,7 @@ var extensionTests = []struct {
 			"y-field": &yExtension{},
 		},
 		layers: []*inputLayer{
-			&inputLayer{
+			{
 				order: 1,
 				label: "layer-xy",
 				yaml: `
@@ -83,7 +81,7 @@ var extensionTests = []struct {
 				`,
 			},
 		},
-		errorString: "cannot parse layer .*: unknown section .*",
+		error: "cannot parse layer .*: unknown section .*",
 	},
 	// Index 3: Load file layers not unique order
 	{
@@ -92,7 +90,7 @@ var extensionTests = []struct {
 			"y-field": &yExtension{},
 		},
 		layers: []*inputLayer{
-			&inputLayer{
+			{
 				order: 1,
 				label: "layer-1",
 				yaml: `
@@ -100,7 +98,7 @@ var extensionTests = []struct {
 					description: desc
 				`,
 			},
-			&inputLayer{
+			{
 				order: 1,
 				label: "layer-2",
 				yaml: `
@@ -109,7 +107,7 @@ var extensionTests = []struct {
 				`,
 			},
 		},
-		errorString: "invalid layer filename: .* not unique .*",
+		error: "invalid layer filename: .* not unique .*",
 	},
 	// Index 4: Load file layers not unique label
 	{
@@ -118,7 +116,7 @@ var extensionTests = []struct {
 			"y-field": &yExtension{},
 		},
 		layers: []*inputLayer{
-			&inputLayer{
+			{
 				order: 1,
 				label: "layer-xy",
 				yaml: `
@@ -126,7 +124,7 @@ var extensionTests = []struct {
 					description: desc
 				`,
 			},
-			&inputLayer{
+			{
 				order: 2,
 				label: "layer-xy",
 				yaml: `
@@ -135,7 +133,7 @@ var extensionTests = []struct {
 				`,
 			},
 		},
-		errorString: "invalid layer filename: .* not unique .*",
+		error: "invalid layer filename: .* not unique .*",
 	},
 	// Index 5: Load file layers with empty section
 	{
@@ -144,7 +142,7 @@ var extensionTests = []struct {
 			"y-field": &yExtension{},
 		},
 		layers: []*inputLayer{
-			&inputLayer{
+			{
 				order: 1,
 				label: "layer-x",
 				yaml: `
@@ -152,7 +150,7 @@ var extensionTests = []struct {
 					description: desc-x
 				`,
 			},
-			&inputLayer{
+			{
 				order: 2,
 				label: "layer-y",
 				yaml: `
@@ -161,11 +159,11 @@ var extensionTests = []struct {
 				`,
 			},
 		},
-		combinedPlanResult: &planResult{
+		result: &planResult{
 			x: &xSection{},
 			y: &ySection{},
 		},
-		combinedPlanResultYaml: string("{}\n"),
+		resultYaml: "{}\n",
 	},
 	// Index 6: Load file layers with section validation failure #1
 	{
@@ -174,7 +172,7 @@ var extensionTests = []struct {
 			"y-field": &yExtension{},
 		},
 		layers: []*inputLayer{
-			&inputLayer{
+			{
 				order: 1,
 				label: "layer-x",
 				yaml: `
@@ -188,7 +186,7 @@ var extensionTests = []struct {
 				`,
 			},
 		},
-		errorString: "cannot validate layer section .* cannot accept entry not starting .*",
+		error: "cannot validate layer section .* cannot accept entry not starting .*",
 	},
 	// Index 7: Load file layers with section validation failure #2
 	{
@@ -197,7 +195,7 @@ var extensionTests = []struct {
 			"y-field": &yExtension{},
 		},
 		layers: []*inputLayer{
-			&inputLayer{
+			{
 				order: 1,
 				label: "layer-x",
 				yaml: `
@@ -208,7 +206,7 @@ var extensionTests = []struct {
 				`,
 			},
 		},
-		errorString: "cannot validate layer section .* cannot have nil entry .*",
+		error: "cannot validate layer section .* cannot have nil entry .*",
 	},
 	// Index 8: Load file layers failed plan validation
 	{
@@ -217,7 +215,7 @@ var extensionTests = []struct {
 			"y-field": &yExtension{},
 		},
 		layers: []*inputLayer{
-			&inputLayer{
+			{
 				order: 1,
 				label: "layer-x",
 				yaml: `
@@ -232,7 +230,7 @@ var extensionTests = []struct {
 							  - y2
 				`,
 			},
-			&inputLayer{
+			{
 				order: 2,
 				label: "layer-y",
 				yaml: `
@@ -246,7 +244,7 @@ var extensionTests = []struct {
 				`,
 			},
 		},
-		errorString: "cannot validate plan section .* cannot find .* as required by .*",
+		error: "cannot validate plan section .* cannot find .* as required by .*",
 	},
 	// Index 9: Check empty section omits entry
 	{
@@ -255,7 +253,7 @@ var extensionTests = []struct {
 			"y-field": &yExtension{},
 		},
 		layers: []*inputLayer{
-			&inputLayer{
+			{
 				order: 1,
 				label: "layer-x",
 				yaml: `
@@ -264,7 +262,7 @@ var extensionTests = []struct {
 					x-field:
 				`,
 			},
-			&inputLayer{
+			{
 				order: 2,
 				label: "layer-y",
 				yaml: `
@@ -274,12 +272,11 @@ var extensionTests = []struct {
 				`,
 			},
 		},
-		combinedPlanResult: &planResult{
+		result: &planResult{
 			x: &xSection{},
 			y: &ySection{},
 		},
-		combinedPlanResultYaml: string(reindent(`
-			{}`)),
+		resultYaml: "{}\n",
 	},
 	// Index 10: Load file layers
 	{
@@ -288,7 +285,7 @@ var extensionTests = []struct {
 			"y-field": &yExtension{},
 		},
 		layers: []*inputLayer{
-			&inputLayer{
+			{
 				order: 1,
 				label: "layer-x",
 				yaml: `
@@ -303,7 +300,7 @@ var extensionTests = []struct {
 							  - y1
 				`,
 			},
-			&inputLayer{
+			{
 				order: 2,
 				label: "layer-y",
 				yaml: `
@@ -317,7 +314,7 @@ var extensionTests = []struct {
 				`,
 			},
 		},
-		combinedPlanResult: &planResult{
+		result: &planResult{
 			x: &xSection{
 				Entries: map[string]*X{
 					"x1": &X{
@@ -342,7 +339,7 @@ var extensionTests = []struct {
 				},
 			},
 		},
-		combinedPlanResultYaml: string(reindent(`
+		resultYaml: string(reindent(`
 			x-field:
 				x1:
 					override: replace
@@ -374,16 +371,16 @@ func (s *S) TestPlanExtensions(c *C) {
 
 		// Load the plan layer from disk (parse, combine and validate).
 		p, err := plan.ReadDir(layersDir)
-		if planTest.errorString != "" || err != nil {
+		if planTest.error != "" || err != nil {
 			// Expected error.
-			c.Assert(err, ErrorMatches, planTest.errorString)
+			c.Assert(err, ErrorMatches, planTest.error)
 		} else {
 			if _, ok := planTest.extensions[xField]; ok {
 				// Verify "x-field" data.
 				var x *xSection
 				x = p.Section(xField).(*xSection)
 				c.Assert(err, IsNil)
-				c.Assert(x.Entries, DeepEquals, planTest.combinedPlanResult.x.Entries)
+				c.Assert(x.Entries, DeepEquals, planTest.result.x.Entries)
 			}
 
 			if _, ok := planTest.extensions[yField]; ok {
@@ -391,13 +388,13 @@ func (s *S) TestPlanExtensions(c *C) {
 				var y *ySection
 				y = p.Section(yField).(*ySection)
 				c.Assert(err, IsNil)
-				c.Assert(y.Entries, DeepEquals, planTest.combinedPlanResult.y.Entries)
+				c.Assert(y.Entries, DeepEquals, planTest.result.y.Entries)
 			}
 
 			// Verify combined plan YAML.
 			planYAML, err := yaml.Marshal(p)
 			c.Assert(err, IsNil)
-			c.Assert(string(planYAML), Equals, planTest.combinedPlanResultYaml)
+			c.Assert(string(planYAML), Equals, planTest.resultYaml)
 		}
 
 		// Unregister test extensions.
diff --git a/internals/plan/plan.go b/internals/plan/plan.go
index fae05339c..b6ddc12ac 100644
--- a/internals/plan/plan.go
+++ b/internals/plan/plan.go
@@ -50,10 +50,10 @@ type LayerSectionExtension interface {
 }
 
 type LayerSection interface {
-	// Ask the section to validate itself.
+	// Validate checks whether the section is valid, returning an error if not.
 	Validate() error
 
-	// Returns true if the section is empty.
+	// IsZero reports whether the section is empty.
 	IsZero() bool
 }
 
@@ -72,12 +72,11 @@ var layerExtensions = map[string]LayerSectionExtension{}
 
 // RegisterExtension adds a plan schema extension. All registrations must be
 // done before the plan library is used.
-func RegisterExtension(field string, ext LayerSectionExtension) error {
+func RegisterExtension(field string, ext LayerSectionExtension) {
 	if _, ok := layerExtensions[field]; ok {
-		return fmt.Errorf("internal error: extension %q already registered", field)
+		panic(fmt.Sprintf("internal error: extension %q already registered", field))
 	}
 	layerExtensions[field] = ext
-	return nil
 }
 
 // UnregisterExtension removes a plan schema extension. This is only
@@ -106,22 +105,22 @@ func (p *Plan) Section(field string) LayerSection {
 // This is required since Sections are based on an inlined map, for which
 // omitempty and inline together is not currently supported.
 func (p *Plan) MarshalYAML() (interface{}, error) {
-	marshalData := make(map[string]interface{})
+	data := make(map[string]interface{})
 	if len(p.Services) != 0 {
-		marshalData["services"] = p.Services
+		data["services"] = p.Services
 	}
 	if len(p.LogTargets) != 0 {
-		marshalData["log-targets"] = p.LogTargets
+		data["log-targets"] = p.LogTargets
 	}
 	if len(p.Checks) != 0 {
-		marshalData["checks"] = p.Checks
+		data["checks"] = p.Checks
 	}
 	for field, section := range p.Sections {
 		if !section.IsZero() {
-			marshalData[field] = section
+			data[field] = section
 		}
 	}
-	return marshalData, nil
+	return data, nil
 }
 
 type Layer struct {
@@ -654,7 +653,7 @@ func CombineLayers(layers ...*Layer) (*Layer, error) {
 		combined.Sections[field], err = extension.CombineSections(sections...)
 		if err != nil {
 			return nil, &FormatError{
-				Message: fmt.Sprintf(`cannot combine section %q: %v`, field, err),
+				Message: fmt.Sprintf("cannot combine section %q: %v", field, err),
 			}
 		}
 	}
@@ -1259,15 +1258,8 @@ func ParseLayer(order int, label string, data []byte) (*Layer, error) {
 				}
 			}
 		} else {
-			if extension, ok := layerExtensions[field]; ok {
-				// Section unmarshal rules are defined by the extension itself.
-				layer.Sections[field], err = extension.ParseSection(section)
-				if err != nil {
-					return nil, &FormatError{
-						Message: fmt.Sprintf("cannot parse layer %q section %q: %v", label, field, err),
-					}
-				}
-			} else {
+			extension, ok := layerExtensions[field]
+			if !ok {
 				// At the top level we do not ignore keys we do not understand.
 				// This preserves the current Pebble behaviour of decoding with
 				// KnownFields = true.
@@ -1275,6 +1267,14 @@ func ParseLayer(order int, label string, data []byte) (*Layer, error) {
 					Message: fmt.Sprintf("cannot parse layer %q: unknown section %q", label, field),
 				}
 			}
+
+			// Section unmarshal rules are defined by the extension itself.
+			layer.Sections[field], err = extension.ParseSection(section)
+			if err != nil {
+				return nil, &FormatError{
+					Message: fmt.Sprintf("cannot parse layer %q section %q: %v", label, field, err),
+				}
+			}
 		}
 	}