diff --git a/src/internal/types/errors/codes.go b/src/internal/types/errors/codes.go
index a09b590352..8c0273571f 100644
--- a/src/internal/types/errors/codes.go
+++ b/src/internal/types/errors/codes.go
@@ -31,6 +31,11 @@ type Code int
 // problem with types.
 
 const (
+	// InvalidSyntaxTree occurs if an invalid syntax tree is provided
+	// to the type checker. It should never happen.
+	InvalidSyntaxTree Code = -1
+
+	// The zero Code value indicates an unset (invalid) error code.
 	_ Code = iota
 
 	// Test is reserved for errors that only apply while in self-test mode.
@@ -40,6 +45,9 @@ const (
 	//
 	// Per the spec:
 	//  "The PackageName must not be the blank identifier."
+	//
+	// Example:
+	//  package _
 	BlankPkgName
 
 	// MismatchedPkgName occurs when a file's package name doesn't match the
diff --git a/src/internal/types/errors/codes_test.go b/src/internal/types/errors/codes_test.go
index 3bf466aec4..6f671a94c6 100644
--- a/src/internal/types/errors/codes_test.go
+++ b/src/internal/types/errors/codes_test.go
@@ -24,7 +24,7 @@ func TestErrorCodeExamples(t *testing.T) {
 			doc := spec.Doc.Text()
 			examples := strings.Split(doc, "Example:")
 			for i := 1; i < len(examples); i++ {
-				example := examples[i]
+				example := strings.TrimSpace(examples[i])
 				err := checkExample(t, example)
 				if err == nil {
 					t.Fatalf("no error in example #%d", i)
@@ -89,8 +89,10 @@ func readCode(err Error) int {
 func checkExample(t *testing.T, example string) error {
 	t.Helper()
 	fset := token.NewFileSet()
-	src := fmt.Sprintf("package p\n\n%s", example)
-	file, err := parser.ParseFile(fset, "example.go", src, 0)
+	if !strings.HasPrefix(example, "package") {
+		example = "package p\n\n" + example
+	}
+	file, err := parser.ParseFile(fset, "example.go", example, 0)
 	if err != nil {
 		t.Fatal(err)
 	}