diff --git a/src/classes.ts b/src/classes.ts
index dbd6429..b7c3540 100644
--- a/src/classes.ts
+++ b/src/classes.ts
@@ -5,7 +5,7 @@ import type {
 import { isPlainObject } from '@metamask/utils';
 import safeStringify from 'fast-safe-stringify';
 
-import type { DataWithOptionalCause } from './utils';
+import type { DataWithOptionalCause, OptionalDataWithOptionalCause } from './utils';
 import { serializeCause } from './utils';
 
 export type { SerializedJsonRpcError };
@@ -16,7 +16,7 @@ export type { SerializedJsonRpcError };
  *
  * Permits any integer error code.
  */
-export class JsonRpcError<Data extends DataWithOptionalCause> extends Error {
+export class JsonRpcError<Data extends OptionalDataWithOptionalCause> extends Error {
   public code: number;
 
   public data?: Data;
diff --git a/src/utils.ts b/src/utils.ts
index 192d7a8..40a4677 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -29,6 +29,16 @@ export type DataWithOptionalCause =
       cause: unknown;
     };
 
+/**
+ * A data object, that must be either:
+ *
+ * - A valid DataWithOptionalCause value.
+ * - undefined.
+ */
+export type OptionalDataWithOptionalCause =
+  | undefined
+  | DataWithOptionalCause;
+
 const FALLBACK_ERROR_CODE = errorCodes.rpc.internal;
 const FALLBACK_MESSAGE =
   'Unspecified error message. This is a bug, please report it.';