Raise an exception if there are no threads.

lib/async/worker_pool.rb

@@ -88,6 +88,10 @@ def close
 		#
 		# @parameter work [Proc] The work to be done.
 		def call(work)
+			if @threads.empty?
+				raise RuntimeError, "No threads available!"
+			end
+
 			handle = Handle.new(::Fiber.scheduler, ::Fiber.current, work)
 
 			begin

test/async/worker_pool.rb

@@ -27,4 +27,14 @@
 			end)
 		end.to raise_exception(ArgumentError, message: be == "Oops!")
 	end
+
+	with "#close" do
+		it "can be closed" do
+			worker_pool.close
+
+			expect do
+				worker_pool.call(proc{})
+			end.to raise_exception(RuntimeError)
+		end
+	end
 end