diff --git a/lib/fugit/cron.rb b/lib/fugit/cron.rb
index 47fa294..13e5dce 100644
--- a/lib/fugit/cron.rb
+++ b/lib/fugit/cron.rb
@@ -46,7 +46,20 @@ def parse(s)
       def do_parse(s)
 
         parse(s) ||
-        fail(ArgumentError.new("invalid cron string #{s.inspect}"))
+        fail(ArgumentError.new("invalid cron string #{trunc(s)}"))
+      end
+
+      protected
+
+      def trunc(s)
+
+        if s.is_a?(String)
+          r = s.length > 28 ? s[0, 28] + "... len #{s.length}" : s
+          r.inspect
+        else
+          r = s.inspect
+          r.length > 35 ? s[0, 35] + '...' : r
+        end
       end
     end
 
diff --git a/lib/fugit/nat.rb b/lib/fugit/nat.rb
index e86de31..e11563e 100644
--- a/lib/fugit/nat.rb
+++ b/lib/fugit/nat.rb
@@ -7,6 +7,8 @@ module Fugit
   #
   module Nat
 
+    MAX_INPUT_LENGTH = 256
+
     class << self
 
       def parse(s, opts={})
@@ -17,6 +19,16 @@ def parse(s, opts={})
 
         s = s.strip
 
+        if s.length > MAX_INPUT_LENGTH
+
+          fail ArgumentError.new(
+            "input too long for a nat string, " +
+            "#{s.length} > #{MAX_INPUT_LENGTH}"
+          ) if opts[:do_parse]
+
+          return nil
+        end
+
 #p s; Raabro.pp(Parser.parse(s, debug: 3), colours: true)
 #(p s; Raabro.pp(Parser.parse(s, debug: 1), colours: true)) rescue nil
 
@@ -29,7 +41,7 @@ def parse(s, opts={})
 
       def do_parse(s, opts={})
 
-        parse(s, opts) ||
+        parse(s, opts.merge(do_parse: true)) ||
         fail(ArgumentError.new("could not parse a nat #{s.inspect}"))
       end
     end
diff --git a/lib/fugit/parse.rb b/lib/fugit/parse.rb
index 73977a7..a8f0552 100644
--- a/lib/fugit/parse.rb
+++ b/lib/fugit/parse.rb
@@ -20,10 +20,10 @@ def parse(s, opts={})
 
       opts[:at] = opts[:in] if opts.has_key?(:in)
 
-      (opts[:cron] != false && parse_cron(s)) ||
-      (opts[:duration] != false && parse_duration(s)) ||
-      (opts[:nat] != false && parse_nat(s, opts)) ||
-      (opts[:at] != false && parse_at(s)) ||
+      (opts[:cron] != false && parse_cron(s)) || # 542ms 616ms
+      (opts[:duration] != false && parse_duration(s)) || # 645ms # 534ms
+      (opts[:nat] != false && parse_nat(s, opts)) || # 2s # 35s
+      (opts[:at] != false && parse_at(s)) || # 568ms 622ms
       nil
     end
 
diff --git a/spec/nat_spec.rb b/spec/nat_spec.rb
index 0c1c08f..e3be77a 100644
--- a/spec/nat_spec.rb
+++ b/spec/nat_spec.rb
@@ -362,12 +362,18 @@
 
     ].each do |input|
 
-      it "fails with ArgumentError for #{input.inspect}" do
+      it "fails with an ArgumentError for #{input.inspect}" do
 
         expect { Fugit::Nat.do_parse(input)
-          }.to raise_error(ArgumentError)
+          }.to raise_error(ArgumentError, /could not parse a nat/)
       end
     end
+
+    it "fails with an ArgumentError when the input length > 256" do
+
+        expect { Fugit::Nat.do_parse('a' * 5000)
+          }.to raise_error(ArgumentError, /too long .+ 5000 > 256/)
+    end
   end
 end