diff --git a/lib/concurrent-ruby/concurrent/promises.rb b/lib/concurrent-ruby/concurrent/promises.rb
index baad27d7d..e1a51b0aa 100644
--- a/lib/concurrent-ruby/concurrent/promises.rb
+++ b/lib/concurrent-ruby/concurrent/promises.rb
@@ -726,7 +726,7 @@ def touched?
       # For inspection.
       # @!visibility private
       def waiting_threads
-        @Waiters.each.to_a
+        @Waiters.value
       end
 
       # @!visibility private
diff --git a/spec/concurrent/promises_spec.rb b/spec/concurrent/promises_spec.rb
index 4fd5df976..96b9e5ba0 100644
--- a/spec/concurrent/promises_spec.rb
+++ b/spec/concurrent/promises_spec.rb
@@ -607,6 +607,19 @@ def behaves_as_delay(delay, value)
   end
 
   describe 'ResolvableEvent' do
+    specify "#waiting_threads" do
+      event = resolvable_event
+      expect(event.waiting_threads).to eq 0
+
+      waiter = Thread.new { event.wait }
+      Thread.pass until waiter.status == "sleep"
+
+      expect(event.waiting_threads).to eq 1
+    ensure
+      event&.resolve(false)
+      waiter&.join
+    end
+
     specify "#wait" do
       event = resolvable_event
       expect(event.wait(0, false)).to be_falsey