diff --git a/lib/httpx/plugins/stream_bidi.rb b/lib/httpx/plugins/stream_bidi.rb
index d218a77f..b497d54e 100644
--- a/lib/httpx/plugins/stream_bidi.rb
+++ b/lib/httpx/plugins/stream_bidi.rb
@@ -94,6 +94,7 @@ module HTTPX
           @oob_buffer = "".b
         end
 
+        # buffers the +chunk+ to be sent
         def <<(chunk)
           return super if Thread.current == @parent_thread
 
@@ -111,29 +112,39 @@ module HTTPX
         end
       end
 
-      # Functions as a way to wake up the session main loop when one of the connections has
-      # buffered data to write. It abides by the Selectable API, which allows it to be
-      # registered in the selector alongside actual HTTP-based Connection objects.
+      # Proxy to wake up the session main loop when one
+      # of the connections has buffered data to write. It abides by the HTTPX::_Selectable API,
+      # which allows it to be registered in the selector alongside actual HTTP-based
+      # HTTPX::Connection objects.
       class Signal
         def initialize
+          @closed = false
           @pipe_read, @pipe_write = ::IO.pipe
         end
 
-        def state; end
+        def state
+          @closed ? :closed : :open
+        end
 
         def to_io
           @pipe_read.to_io
         end
 
         def wakeup
+          return if @closed
+
           @pipe_write.write("\0")
         end
 
         def call
+          return if @closed
+
           @pipe_read.readpartial(1)
         end
 
         def interests
+          return if @closed
+
           :r
         end
 
@@ -142,6 +153,7 @@ module HTTPX
         def terminate
           @pipe_write.close
           @pipe_read.close
+          @closed = true
         end
       end
 
diff --git a/sig/plugins/stream_bidi.rbs b/sig/plugins/stream_bidi.rbs
index 72013b17..547dbe68 100644
--- a/sig/plugins/stream_bidi.rbs
+++ b/sig/plugins/stream_bidi.rbs
@@ -24,6 +24,7 @@ module HTTPX
       end
 
       class Signal
+        @closed: bool
         @pipe_read: ::IO
         @pipe_write: ::IO
 
diff --git a/sig/selector.rbs b/sig/selector.rbs
index 942dfa8d..31925e14 100644
--- a/sig/selector.rbs
+++ b/sig/selector.rbs
@@ -1,6 +1,6 @@
 module HTTPX
   interface _Selectable
-    def state: () -> Symbol?
+    def state: () -> Symbol
 
     def to_io: () -> ::IO