diff --git a/lib/timeout.rb b/lib/timeout.rb
index 6448f4f..a1595a8 100644
--- a/lib/timeout.rb
+++ b/lib/timeout.rb
@@ -141,9 +141,10 @@ def self.ensure_timeout_thread_created
   # Perform an operation in a block, raising an error if it takes longer than
   # +sec+ seconds to complete.
   #
-  # +sec+:: Number of seconds to wait for the block to terminate. Any number
-  #         may be used, including Floats to specify fractional seconds. A
+  # +sec+:: Number of seconds to wait for the block to terminate. Any non-negative number
+  #         or nil may be used, including Floats to specify fractional seconds. A
   #         value of 0 or +nil+ will execute the block without any timeout.
+  #         Any negative number will raise an ArgumentError.
   # +klass+:: Exception Class to raise if the block fails to terminate
   #           in +sec+ seconds.  Omitting will use the default, Timeout::Error
   # +message+:: Error message to raise with Exception Class.
@@ -165,6 +166,7 @@ def self.ensure_timeout_thread_created
   # a module method, so you can call it directly as Timeout.timeout().
   def timeout(sec, klass = nil, message = nil, &block)   #:yield: +sec+
     return yield(sec) if sec == nil or sec.zero?
+    raise ArgumentError, "Timeout sec must be a non-negative number" if 0 > sec
 
     message ||= "execution expired"
 
diff --git a/test/test_timeout.rb b/test/test_timeout.rb
index 34966f9..0115686 100644
--- a/test/test_timeout.rb
+++ b/test/test_timeout.rb
@@ -31,6 +31,12 @@ def test_allows_nil_seconds
     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
@@ -114,7 +120,7 @@ def test_nested_timeout_which_error_bubbles_up
   def test_cannot_convert_into_time_interval
     bug3168 = '[ruby-dev:41010]'
     def (n = Object.new).zero?; false; end
-    assert_raise(TypeError, bug3168) {Timeout.timeout(n) { sleep 0.1 }}
+    assert_raise(ArgumentError, bug3168) {Timeout.timeout(n) { sleep 0.1 }}
   end
 
   def test_skip_rescue_standarderror