diff --git a/lib/http.mjs b/lib/http.mjs
index c5a7778..5d1bf54 100644
--- a/lib/http.mjs
+++ b/lib/http.mjs
@@ -43,6 +43,7 @@ const httpOpts = new Map([
   [403, { status: 403, statusText: 'Forbidden' }],
   [404, { status: 404, statusText: 'Not Found' }],
   [405, { status: 405, statusText: 'Method Not Allowed', headers: HEADERS_ALLOW }],
+  [412, { status: 412, statusText: 'Precondition Failed' }],
   [416, { status: 416, statusText: 'Range Not Satisfiable' }],
   [422, { status: 422, statusText: 'Unprocessable Content' }],
   [500, { status: 500, statusText: 'Internal Server Error' }],
diff --git a/lib/methods.mjs b/lib/methods.mjs
index 724fa6d..21f9e15 100644
--- a/lib/methods.mjs
+++ b/lib/methods.mjs
@@ -43,6 +43,17 @@ const GET = async (url, { headers, integrity, signal }) => {
       await statsAsOptions(stats, { status: 304 })
     );
 
+  if (headers.has('If-Match') &&
+      !headers.get('If-Match')
+        .split(',')
+        .map($ => $.trim())
+        .includes(computeWeakETag(stats))
+  )
+    return new Response(
+      null,
+      await statsAsOptions(stats, { status: 412 })
+    );
+
   const ranges = getRanges(headers);
 
   return ranges.length