diff --git a/core/indexing/chunk/markdown.ts b/core/indexing/chunk/markdown.ts
index 5a0df6776d8..b3b3765fdf9 100644
--- a/core/indexing/chunk/markdown.ts
+++ b/core/indexing/chunk/markdown.ts
@@ -133,7 +133,9 @@ export async function* markdownChunker(
       hLevel + 1,
     )) {
       yield {
-        content: `${section.header}\n${chunk.content}`,
+        content: section.header
+          ? `${section.header}\n${chunk.content}`
+          : chunk.content,
         startLine: section.startLine + chunk.startLine,
         endLine: section.startLine + chunk.endLine,
         otherMetadata: {
diff --git a/core/indexing/chunk/markdown.vitest.ts b/core/indexing/chunk/markdown.vitest.ts
new file mode 100644
index 00000000000..e7eb791b8f7
--- /dev/null
+++ b/core/indexing/chunk/markdown.vitest.ts
@@ -0,0 +1,77 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("../../llm/countTokens", () => ({
+  countTokens: (content: string) => content.length,
+  countTokensAsync: async (content: string) => content.length,
+}));
+
+import { ChunkWithoutID } from "../../index";
+
+import { markdownChunker } from "./markdown";
+
+async function collectChunks(
+  content: string,
+  maxChunkSize: number,
+  hLevel: number,
+): Promise<ChunkWithoutID[]> {
+  const chunks: ChunkWithoutID[] = [];
+  for await (const chunk of markdownChunker(content, maxChunkSize, hLevel)) {
+    chunks.push(chunk);
+  }
+  return chunks;
+}
+
+describe("markdownChunker", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("should not prefix chunks with 'undefined' when a section has no header", async () => {
+    const content = Array.from(
+      { length: 10 },
+      (_, i) => `Line ${i} of a markdown document that has no headers at all.`,
+    ).join("\n");
+
+    const chunks = await collectChunks(content, 100, 0);
+
+    expect(chunks.length).toBeGreaterThan(0);
+    for (const chunk of chunks) {
+      expect(chunk.content).not.toContain("undefined");
+    }
+  });
+
+  it("should not prefix headerless sub-sections with 'undefined'", async () => {
+    const content = [
+      "## Installation",
+      ...Array.from(
+        { length: 10 },
+        (_, i) => `Installation step ${i} with a bit of extra detail.`,
+      ),
+    ].join("\n");
+
+    const chunks = await collectChunks(content, 120, 1);
+
+    expect(chunks.length).toBeGreaterThan(0);
+    for (const chunk of chunks) {
+      expect(chunk.content).not.toContain("undefined");
+      expect(chunk.content.startsWith("## Installation\n")).toBe(true);
+    }
+  });
+
+  it("should still prepend the section header to each chunk", async () => {
+    const content = [
+      "## Installation",
+      ...Array.from(
+        { length: 10 },
+        (_, i) => `Installation step ${i} with a bit of extra detail.`,
+      ),
+    ].join("\n");
+
+    const chunks = await collectChunks(content, 120, 1);
+
+    expect(chunks.length).toBeGreaterThan(1);
+    for (const chunk of chunks) {
+      expect(chunk.otherMetadata?.title).toBe("Installation");
+    }
+  });
+});