diff --git a/src/validator.ts b/src/validator.ts
index 096c7b7..8b9b80e 100644
--- a/src/validator.ts
+++ b/src/validator.ts
@@ -208,7 +208,9 @@ export async function validateRequest(
 
     // Check Content-Type matches one of the expected media types
     const expectedTypes = Object.keys(rb.content);
-    const baseContentType = contentType.split(";")[0].trim().toLowerCase();
+    const baseContentType =
+      contentType.split(";")[0].trim().toLowerCase() ||
+      "application/octet-stream";
 
     const selectedMediaType = selectMediaType(rb.content, baseContentType);
     if (baseContentType && expectedTypes.length > 0 && !selectedMediaType) {
@@ -222,7 +224,11 @@ export async function validateRequest(
     }
 
     // Validate body structure for JSON
-    if (baseContentType === "application/json" && selectedMediaType?.schema) {
+    if (
+      (baseContentType === "application/json" ||
+        baseContentType.endsWith("+json")) &&
+      selectedMediaType?.schema
+    ) {
       try {
         const cloned = req.clone();
         const body: unknown = await cloned.json();
@@ -314,7 +320,9 @@ export function validateResponse(
     matched.operation.responses.default;
   if (responseSpec?.content) {
     const contentType = res.headers.get("content-type") || "";
-    const baseContentType = contentType.split(";")[0].trim().toLowerCase();
+    const baseContentType =
+      contentType.split(";")[0].trim().toLowerCase() ||
+      (res.body ? "application/octet-stream" : "");
     const expectedTypes = Object.keys(responseSpec.content);
 
     if (
diff --git a/tests/nov4-regression.test.ts b/tests/nov4-regression.test.ts
new file mode 100644
index 0000000..decbf78
--- /dev/null
+++ b/tests/nov4-regression.test.ts
@@ -0,0 +1,75 @@
+import assert from "node:assert/strict";
+import { it } from "node:test";
+import type { MatchedOperation } from "../src/types.js";
+import { validateRequest, validateResponse } from "../src/validator.js";
+
+function target(media = "application/json"): MatchedOperation {
+  return {
+    operation: {
+      requestBody: {
+        content: { [media]: { schema: { required: ["id"], type: "object" } } },
+      },
+      responses: { "200": { content: { [media]: {} }, description: "OK" } },
+    },
+    pathParams: {},
+    pathTemplate: "/items",
+  };
+}
+
+it("validates JSON syntax and fields for structured JSON media types", async () => {
+  await Promise.all(
+    ["{}", "not json"].map(async (body) => {
+      const req = new Request("https://example.com/items", {
+        body,
+        headers: { "content-type": "Application/Problem+JSON; charset=utf-8" },
+        method: "POST",
+      });
+      const violations = await validateRequest(
+        req,
+        target("application/problem+json"),
+        "/items"
+      );
+      assert.equal(violations.length, 1);
+      assert.equal(
+        violations[0].issue,
+        body === "{}"
+          ? "Missing required field: id"
+          : "Request body is not valid JSON"
+      );
+    })
+  );
+  const req = new Request("https://example.com/items", {
+    body: '{"id":1}',
+    headers: { "content-type": "application/problem+json" },
+    method: "POST",
+  });
+  assert.deepEqual(
+    await validateRequest(req, target("application/problem+json"), "/items"),
+    []
+  );
+});
+
+it("checks the effective media type of bodies without Content-Type", async () => {
+  const bytes = new TextEncoder().encode("{}");
+  const req = new Request("https://example.com/items", {
+    body: bytes,
+    method: "POST",
+  });
+  const res = new Response(bytes);
+  assert.equal(req.headers.has("content-type"), false);
+  assert.equal(res.headers.has("content-type"), false);
+  assert.equal((await validateRequest(req, target(), "/items")).length, 1);
+  assert.equal(
+    (await validateResponse(req, res, target(), "/items")).length,
+    1
+  );
+  assert.deepEqual(await validateRequest(req, target("*/*"), "/items"), []);
+  assert.deepEqual(
+    await validateResponse(req, res, target("*/*"), "/items"),
+    []
+  );
+  assert.deepEqual(
+    await validateResponse(req, new Response(null), target(), "/items"),
+    []
+  );
+});