diff --git a/.changeset/tough-rooms-camp.md b/.changeset/tough-rooms-camp.md
new file mode 100644
index 00000000000..26ae61b4f83
--- /dev/null
+++ b/.changeset/tough-rooms-camp.md
@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Reject NDJSON values without a JSON representation.
diff --git a/packages/effect/src/unstable/encoding/Ndjson.ts b/packages/effect/src/unstable/encoding/Ndjson.ts
index 87bcce19746..ce9e33b424f 100644
--- a/packages/effect/src/unstable/encoding/Ndjson.ts
+++ b/packages/effect/src/unstable/encoding/Ndjson.ts
@@ -73,7 +73,15 @@ export const encodeString = <IE = never, Done = unknown>(): Channel.Channel<
   Channel.fromTransform((upstream, _scope) =>
     Effect.succeed(Effect.flatMap(upstream, (input) => {
       try {
-        return Effect.succeed(Arr.of(input.map((item) => JSON.stringify(item)).join("\n") + "\n"))
+        return Effect.succeed(Arr.of(
+          input.map((item) => {
+            const output = JSON.stringify(item)
+            if (output === undefined) {
+              throw new TypeError("Value cannot be represented as JSON")
+            }
+            return output
+          }).join("\n") + "\n"
+        ))
       } catch (cause) {
         return Effect.fail(new NdjsonError({ kind: "Pack", cause }))
       }
diff --git a/packages/effect/test/unstable/encoding/Ndjson.test.ts b/packages/effect/test/unstable/encoding/Ndjson.test.ts
index 4e1f06d558e..fbc0b10ce36 100644
--- a/packages/effect/test/unstable/encoding/Ndjson.test.ts
+++ b/packages/effect/test/unstable/encoding/Ndjson.test.ts
@@ -4,6 +4,22 @@ import * as Schema from "effect/Schema"
 import * as Ndjson from "effect/unstable/encoding/Ndjson"
 
 describe("Ndjson", () => {
+  it.effect("fails for values without a JSON representation", () =>
+    Effect.gen(function*() {
+      const inputs = [undefined, () => {}, Symbol("x")]
+
+      for (const input of inputs) {
+        const error = yield* Stream.make(input).pipe(
+          Stream.pipeThroughChannel(Ndjson.encodeString()),
+          Stream.runCollect,
+          Effect.flip
+        )
+
+        assert.instanceOf(error, Ndjson.NdjsonError)
+        assert.strictEqual(error.kind, "Pack")
+      }
+    }))
+
   it.effect("decodeSchema decodes records split across Uint8Array chunks", () =>
     Effect.gen(function*() {
       const messages = yield* Stream.make(