socketry
diff --git a/‎lib/async/scheduler.rb
Lines changed: 9 additions & 1 deletion b/‎lib/async/scheduler.rb
Lines changed: 9 additions & 1 deletion
diff --git a/‎lib/async/task.rb
Lines changed: 7 additions & 1 deletion b/‎lib/async/task.rb
Lines changed: 7 additions & 1 deletion
diff --git a/‎test/async/reactor.rb
Lines changed: 13 additions & 0 deletions b/‎test/async/reactor.rb
Lines changed: 13 additions & 0 deletions
@@ -17,6 +17,12 @@
 module Async
 	# Handles scheduling of fibers. Implements the fiber scheduler interface.
 	class Scheduler < Node
+		class ClosedError < RuntimeError
+			def initialize(message = "Scheduler is closed!")
+				super
+			end
+		end
+		
 		# Whether the fiber scheduler is supported.
 		# @public Since `stable-v1`.
 		def self.supported?
@@ -233,7 +239,7 @@ def run_once(timeout = nil)
 
 		# Run the reactor until all tasks are finished. Proxies arguments to {#async} immediately before entering the loop, if a block is provided.
 		def run(...)
-			Kernel::raise RuntimeError, 'Reactor has been closed' if @selector.nil?
+			Kernel::raise ClosedError if @selector.nil?
 
 			initial_task = self.async(...) if block_given?
 
@@ -260,6 +266,8 @@ def run(...)
 		# @returns [Task] The task that was scheduled into the reactor.
 		# @deprecated With no replacement.
 		def async(*arguments, **options, &block)
+			Kernel::raise ClosedError if @selector.nil?
+			
 			task = Task.new(Task.current? || self, **options, &block)
 
 			# I want to take a moment to explain the logic of this.
 
@@ -59,6 +59,12 @@ def initialize(message = "execution expired")
 	#
 	# @public Since `stable-v1`.
 	class Task < Node
+		class FinishedError < RuntimeError
+			def initialize(message = "Cannot create child task within a task that has finished execution!")
+				super
+			end
+		end
+		
 		# @deprecated With no replacement.
 		def self.yield
 			Fiber.scheduler.transfer
@@ -164,7 +170,7 @@ def run(*arguments)
 
 		# Run an asynchronous task as a child of the current task.
 		def async(*arguments, **options, &block)
-			raise "Cannot create child task within a task that has finished execution!" if self.finished?
+			raise FinishedError if self.finished?
 
 			task = Task.new(self, **options, &block)
 
 
@@ -267,4 +267,17 @@
 			expect(reactor.to_s).to be =~ /stopped/
 		end
 	end
+	
+	it "validates scheduler assignment" do
+		# Assign the scheduler:
+		reactor = self.reactor
+		
+		# Close the previous scheduler:
+		Async {}
+		
+		expect do
+			# The reactor is closed:
+			reactor.async {}
+		end.to raise_exception(Async::Scheduler::ClosedError)
+	end
 end