diff --git a/src/error/GraphQLError.ts b/src/error/GraphQLError.ts
index 2bb3ccb7cd..53efd1d326 100644
--- a/src/error/GraphQLError.ts
+++ b/src/error/GraphQLError.ts
@@ -44,7 +44,7 @@ export function isGraphQLError(error: unknown): error is GraphQLError {
  * GraphQL document and/or execution result that correspond to the Error.
  */
 export class GraphQLError extends Error {
-  [isGraphQLErrorSymbol]: true = true;
+  readonly [isGraphQLErrorSymbol]: true = true;
   /**
    * An array of `{ line, column }` locations within the source GraphQL document
    * which correspond to this error.
diff --git a/src/language/source.ts b/src/language/source.ts
index 8f6fed590a..1284da249e 100644
--- a/src/language/source.ts
+++ b/src/language/source.ts
@@ -15,7 +15,7 @@ const isSourceSymbol = Symbol.for('Source');
  * The `line` and `column` properties in `locationOffset` are 1-indexed.
  */
 export class Source {
-  [isSourceSymbol]: true = true;
+  readonly [isSourceSymbol]: true = true;
   body: string;
   name: string;
   locationOffset: Location;
diff --git a/src/type/directives.ts b/src/type/directives.ts
index 440cc432d3..7f1c09a987 100644
--- a/src/type/directives.ts
+++ b/src/type/directives.ts
@@ -57,7 +57,7 @@ const isGraphQLDirectiveSymbol = Symbol.for('GraphQLDirective');
  * behavior. Type system creators will usually not create these directly.
  */
 export class GraphQLDirective {
-  [isGraphQLDirectiveSymbol]: true = true;
+  readonly [isGraphQLDirectiveSymbol]: true = true;
   name: string;
   description: Maybe<string>;
   locations: ReadonlyArray<DirectiveLocation>;
diff --git a/src/type/schema.ts b/src/type/schema.ts
index 61b11a9fbd..ddd81d3b5d 100644
--- a/src/type/schema.ts
+++ b/src/type/schema.ts
@@ -136,11 +136,12 @@ const isSchemaSymbol = Symbol.for('GraphQLSchema');
  * ```
  */
 export class GraphQLSchema {
+  readonly [isSchemaSymbol]: true = true;
+
   description: Maybe<string>;
   extensions: Readonly<GraphQLSchemaExtensions>;
   astNode: Maybe<SchemaDefinitionNode>;
   extensionASTNodes: ReadonlyArray<SchemaExtensionNode>;
-  [isSchemaSymbol]: true = true;
 
   // Used as a cache for validateSchema().
   __validationErrors: Maybe<ReadonlyArray<GraphQLError>>;