diff --git a/lib/optparse.rb b/lib/optparse.rb
index d363b91..fbcd7f9 100644
--- a/lib/optparse.rb
+++ b/lib/optparse.rb
@@ -1639,7 +1639,7 @@ def order(*argv, into: nil, &nonopt)
   # Non-option arguments remain in +argv+.
   #
   def order!(argv = default_argv, into: nil, &nonopt)
-    setter = ->(name, val = nil) {into[name.to_sym] = val} if into
+    setter = ->(name, val) {into[name.to_sym] = val} if into
     parse_in_order(argv, setter, &nonopt)
   end
 
@@ -1665,8 +1665,8 @@ def parse_in_order(argv = default_argv, setter = nil, &nonopt)  # :nodoc:
           end
           begin
             opt, cb, *val = sw.parse(rest, argv) {|*exc| raise(*exc)}
-            val = cb.call(*val) if cb
-            setter.call(sw.switch_name, *val) if setter
+            val = callback!(cb, 1, *val) if cb
+            callback!(setter, 2, sw.switch_name, *val) if setter
           rescue ParseError
             raise $!.set_option(arg, rest)
           end
@@ -1704,8 +1704,8 @@ def parse_in_order(argv = default_argv, setter = nil, &nonopt)  # :nodoc:
           end
           begin
             argv.unshift(opt) if opt and (!rest or (opt = opt.sub(/\A-*/, '-')) != '-')
-            val = cb.call(*val) if cb
-            setter.call(sw.switch_name, *val) if setter
+            val = callback!(cb, 1, *val) if cb
+            callback!(setter, 2, sw.switch_name, *val) if setter
           rescue ParseError
             raise $!.set_option(arg, arg.length > 2)
           end
@@ -1731,6 +1731,17 @@ def parse_in_order(argv = default_argv, setter = nil, &nonopt)  # :nodoc:
   end
   private :parse_in_order
 
+  # Calls callback with _val_.
+  def callback!(cb, max_arity, *args) # :nodoc:
+    if (size = args.size) < max_arity and cb.to_proc.lambda?
+      (arity = cb.arity) < 0 and arity = (1-arity)
+      arity = max_arity if arity > max_arity
+      args[arity - 1] = nil if arity > size
+    end
+    cb.call(*args)
+  end
+  private :callback!
+
   #
   # Parses command line arguments +argv+ in permutation mode and returns
   # list of non-option arguments. When optional +into+ keyword
diff --git a/test/optparse/test_optarg.rb b/test/optparse/test_optarg.rb
index f4882b0..f944605 100644
--- a/test/optparse/test_optarg.rb
+++ b/test/optparse/test_optarg.rb
@@ -10,6 +10,7 @@ def setup
     @opt.def_option "--with_underscore[=VAL]" do |x| @flag = x end
     @opt.def_option "--with-hyphen[=VAL]" do |x| @flag = x end
     @opt.def_option("--fallback[=VAL]") do |x = "fallback"| @flag = x end
+    @opt.def_option("--lambda[=VAL]", &->(x) {@flag = x})
     @reopt = nil
   end
 
@@ -65,4 +66,11 @@ def test_default_argument
     assert_equal(%w"", no_error {@opt.parse!(%w"--fallback")})
     assert_equal("fallback", @flag)
   end
+
+  def test_lambda
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda=lambda1")})
+    assert_equal("lambda1", @flag)
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda")})
+    assert_equal(nil, @flag)
+  end
 end
diff --git a/test/optparse/test_placearg.rb b/test/optparse/test_placearg.rb
index 56b641b..a8a11e6 100644
--- a/test/optparse/test_placearg.rb
+++ b/test/optparse/test_placearg.rb
@@ -14,6 +14,7 @@ def setup
     @opt.def_option "--with_underscore=VAL" do |x| @flag = x end
     @opt.def_option "--with-hyphen=VAL" do |x| @flag = x end
     @opt.def_option("--fallback [VAL]") do |x = "fallback"| @flag = x end
+    @opt.def_option("--lambda [VAL]", &->(x) {@flag = x})
   end
 
   def test_short
@@ -83,4 +84,13 @@ def test_default_argument
     assert_equal(%w"", no_error {@opt.parse!(%w"--fallback")})
     assert_equal("fallback", @flag)
   end
+
+  def test_lambda
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda=lambda1")})
+    assert_equal("lambda1", @flag)
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda lambda2")})
+    assert_equal("lambda2", @flag)
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda")})
+    assert_equal(nil, @flag)
+  end
 end
diff --git a/test/optparse/test_reqarg.rb b/test/optparse/test_reqarg.rb
index d5686d1..31d4fef 100644
--- a/test/optparse/test_reqarg.rb
+++ b/test/optparse/test_reqarg.rb
@@ -6,6 +6,7 @@ def setup
     super
     @opt.def_option "--with_underscore=VAL" do |x| @flag = x end
     @opt.def_option "--with-hyphen=VAL" do |x| @flag = x end
+    @opt.def_option("--lambda=VAL", &->(x) {@flag = x})
   end
 
   class Def1 < TestOptionParser
@@ -81,6 +82,11 @@ def test_hyphenize
     assert_equal("foo4", @flag)
   end
 
+  def test_lambda
+    assert_equal(%w"", no_error {@opt.parse!(%w"--lambda=lambda1")})
+    assert_equal("lambda1", @flag)
+  end
+
   class TestOptionParser::WithPattern < TestOptionParser
     def test_pattern
       pat = num = nil