diff --git a/src/error/GraphQLError.ts b/src/error/GraphQLError.ts
index 80bb9fcba3..59b74e1648 100644
--- a/src/error/GraphQLError.ts
+++ b/src/error/GraphQLError.ts
@@ -61,7 +61,7 @@ export class GraphQLError extends Error {
   /**
    * Extension fields to add to the formatted error.
    */
-  readonly extensions: { [key: string]: unknown } | undefined;
+  readonly extensions: { [key: string]: unknown };
 
   constructor(
     message: string,
@@ -124,7 +124,7 @@ export class GraphQLError extends Error {
       ? originalError?.extensions
       : undefined;
     // TODO: merge `extensions` and `originalExtensions`
-    this.extensions = extensions ?? originalExtensions;
+    this.extensions = extensions ?? originalExtensions ?? Object.create(null);
 
     // Include (non-enumerable) stack trace.
     if (originalError?.stack) {
@@ -183,7 +183,7 @@ export class GraphQLError extends Error {
       formattedError.path = this.path;
     }
 
-    if (this.extensions != null) {
+    if (this.extensions != null && Object.keys(this.extensions).length > 0) {
       formattedError.extensions = this.extensions;
     }
 
diff --git a/src/error/__tests__/GraphQLError-test.ts b/src/error/__tests__/GraphQLError-test.ts
index ca5679710e..dc7a887f5b 100644
--- a/src/error/__tests__/GraphQLError-test.ts
+++ b/src/error/__tests__/GraphQLError-test.ts
@@ -28,10 +28,14 @@ describe('GraphQLError', () => {
     expect(new GraphQLError('str')).to.be.instanceof(GraphQLError);
   });
 
-  it('has a name, message, and stack trace', () => {
+  it('has a name, message, extensions, and stack trace', () => {
     const e = new GraphQLError('msg');
 
-    expect(e).to.include({ name: 'GraphQLError', message: 'msg' });
+    expect(e).to.deep.include({
+      name: 'GraphQLError',
+      message: 'msg',
+      extensions: {},
+    });
     expect(e.stack).to.be.a('string');
   });
 
diff --git a/tsconfig.json b/tsconfig.json
index 7f3fef7346..8dfb0f4794 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -8,6 +8,7 @@
     "useUnknownInCatchVariables": false,
     "noEmit": true,
     "isolatedModules": true,
+    "importsNotUsedAsValues": "error",
     "forceConsistentCasingInFileNames": true
   }
 }