diff --git a/lib/timeout.rb b/lib/timeout.rb
index c8c3d501b6..59437f490e 100644
--- a/lib/timeout.rb
+++ b/lib/timeout.rb
@@ -164,7 +164,7 @@ module Timeout
   # Timeout</tt> into your classes so they have a #timeout method, as well as
   # a module method, so you can call it directly as Timeout.timeout().
   def timeout(sec, klass = nil, message = nil, &block)   #:yield: +sec+
-    raise ArgumentError, "Timeout sec must be a positive number" unless sec.is_a?(Numeric) && sec >= 0
+    raise ArgumentError, "Timeout sec must be a positive number" if sec.is_a?(Numeric) && sec < 0
     return yield(sec) if sec == nil or sec.zero?
 
     message ||= "execution expired"
diff --git a/test/test_timeout.rb b/test/test_timeout.rb
index 34966f92a4..888489f430 100644
--- a/test/test_timeout.rb
+++ b/test/test_timeout.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: false
 require 'test/unit'
-require 'timeout'
+require_relative '../lib/timeout'
 
 class TestTimeout < Test::Unit::TestCase
 
@@ -31,6 +31,12 @@ class TestTimeout < Test::Unit::TestCase
     end
   end
 
+  def test_raise_for_neg_second
+    assert_raise(ArgumentError) do
+      Timeout.timeout(-1) { sleep(0.01) }
+    end
+  end
+
   def test_included
     c = Class.new do
       include Timeout