diff --git a/CHANGELOG.md b/CHANGELOG.md
index 01ce6578..e78ea435 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -40,6 +40,7 @@
   `429 Too Many Requests` for a full remote browser service, and exposes it as `#status`
 
 ### Fixed
+- `workers` skips detached workers, `target.worker` with no session raises `NoSuchTargetError` instead of a CDP error
 - `attach_target` reuses a target's session and returns the target, instead of opening one more that reruns extensions
 - Service workers registered by a page stayed attached and alive for the browser's lifetime, now Chrome can stop them
 - An out-of-process iframe kept its parent page's session when `Target.targetCreated` came before
diff --git a/lib/ferrum/context.rb b/lib/ferrum/context.rb
index 94b0cbd0..c9fe0e6b 100644
--- a/lib/ferrum/context.rb
+++ b/lib/ferrum/context.rb
@@ -56,11 +56,12 @@ def pages
       @targets.values.select(&:page?).map(&:page)
     end
 
-    # Dedicated and shared workers spawned by any page in this context.
+    # Dedicated and shared workers spawned by any page in this context,
+    # leaving out the ones we have no session with, e.g. detached ones.
     #
     # @return [Array<Worker>]
     def workers
-      @targets.values.select { |t| t.worker? || t.shared_worker? }.map(&:worker)
+      @targets.values.select { |t| (t.worker? || t.shared_worker?) && t.session_id }.map(&:worker)
     end
 
     # Service worker targets registered in this context. Unlike {#workers},
diff --git a/lib/ferrum/target.rb b/lib/ferrum/target.rb
index 5fb97fc1..882998b9 100644
--- a/lib/ferrum/target.rb
+++ b/lib/ferrum/target.rb
@@ -55,6 +55,9 @@ def page
     # Connects to and returns this target's {Worker}.
     #
     # @return [Worker]
+    #
+    # @raise [NoSuchTargetError] if there's no session with the target, e.g. a service worker
+    #   not attached to with {Context#attach_target}.
     def worker
       @worker ||= build_worker
     end
@@ -82,6 +85,10 @@ def build_page(**options)
     #
     # @return [Worker]
     def build_worker
+      if options.flatten && session_id.nil?
+        raise NoSuchTargetError, "No session with #{type} #{id}, attach to it with Context#attach_target first"
+      end
+
       Worker.new(client, target_id: id, url: url)
     end
 
diff --git a/spec/worker_spec.rb b/spec/worker_spec.rb
index beb08493..83a4998f 100644
--- a/spec/worker_spec.rb
+++ b/spec/worker_spec.rb
@@ -50,6 +50,23 @@ def wait_for_target(&block)
       wait_for { worker.console.map(&:text) }.to eq(["from worker"])
       expect(worker.console.first).to be_warning
     end
+
+    it "is left out of Context#workers once detached" do
+      page.go_to
+
+      page.execute <<~JS
+        new Worker(URL.createObjectURL(new Blob(
+          ["self.onmessage = () => {}"],
+          { type: "application/javascript" }
+        )))
+      JS
+
+      target = wait_for_target(&:worker?)
+      wait_for { browser.workers }.to contain_exactly(target.worker)
+      page.command("Target.detachFromTarget", sessionId: target.session_id)
+
+      wait_for { browser.workers }.to be_empty
+    end
   end
 
   describe "shared workers" do
@@ -95,6 +112,15 @@ def wait_for_target(&block)
       expect(target.session_id).to be_nil
     end
 
+    it "raises when connected to without attaching first" do
+      page.go_to
+      page.evaluate_async(%(navigator.serviceWorker.register("/sw.js").then(arguments[0])), 5)
+      target = wait_for_target(&:service_worker?)
+      wait_for { target.session_id }.to be_nil
+
+      expect { target.worker }.to raise_error(Ferrum::NoSuchTargetError, /attach_target/)
+    end
+
     it "connects on demand through Context#attach_target, keeping it alive" do
       page.go_to
       page.evaluate_async(%(navigator.serviceWorker.register("/sw.js").then(arguments[0])), 5)