diff --git a/jest.config.js b/jest.config.js
index 0ee455d..0acc5bf 100644
--- a/jest.config.js
+++ b/jest.config.js
@@ -45,10 +45,10 @@ module.exports = {
   // An object that configures minimum threshold enforcement for coverage results
   coverageThreshold: {
     global: {
-      branches: 94.25,
+      branches: 94.31,
       functions: 94.11,
-      lines: 97.04,
-      statements: 97.04,
+      lines: 97.05,
+      statements: 97.05,
     },
   },
 
diff --git a/src/errors.test.ts b/src/errors.test.ts
index 5a5f69c..9e6a24f 100644
--- a/src/errors.test.ts
+++ b/src/errors.test.ts
@@ -118,6 +118,25 @@ describe('rpcErrors', () => {
       },
     });
   });
+
+  it('serializes a non-Error-instance cause', () => {
+    const error = rpcErrors.invalidInput({
+      data: {
+        foo: 'bar',
+        cause: 'foo',
+      },
+    });
+
+    const serializedError = error.serialize();
+    assert(serializedError.data);
+    assert(isPlainObject(serializedError.data));
+
+    expect(serializedError.data.cause).not.toBeInstanceOf(Error);
+    expect(serializedError.data).toStrictEqual({
+      foo: 'bar',
+      cause: 'foo',
+    });
+  });
 });
 
 describe('providerErrors', () => {
@@ -169,4 +188,23 @@ describe('providerErrors', () => {
       },
     });
   });
+
+  it('serializes a non-Error-instance cause', () => {
+    const error = providerErrors.unauthorized({
+      data: {
+        foo: 'bar',
+        cause: 'foo',
+      },
+    });
+
+    const serializedError = error.serialize();
+    assert(serializedError.data);
+    assert(isPlainObject(serializedError.data));
+
+    expect(serializedError.data.cause).not.toBeInstanceOf(Error);
+    expect(serializedError.data).toStrictEqual({
+      foo: 'bar',
+      cause: 'foo',
+    });
+  });
 });
diff --git a/src/utils.ts b/src/utils.ts
index 76ed5d5..c6f28c7 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -14,14 +14,17 @@ import { errorCodes, errorValues } from './error-constants';
  * A data object, that must be either:
  *
  * - A JSON-serializable object.
- * - An object with a `cause` property that is an `Error` instance, and any
+ * - An object with a `cause` property that is an error-like value, and any
  * other properties that are JSON-serializable.
  */
 export type DataWithOptionalCause =
   | Json
   | {
-      [key: string]: Json | Error;
-      cause: Error;
+      // Unfortunately we can't use just `Json` here, because all properties of
+      // an object with an index signature must be assignable to the index
+      // signature's type. So we have to use `Json | unknown` instead.
+      [key: string]: Json | unknown;
+      cause: unknown;
     };
 
 const FALLBACK_ERROR_CODE = errorCodes.rpc.internal;