diff --git a/spec/cbor/lexer_spec.cr b/spec/cbor/lexer_spec.cr
index 07a5d8a..80edc10 100644
--- a/spec/cbor/lexer_spec.cr
+++ b/spec/cbor/lexer_spec.cr
@@ -2,33 +2,35 @@ require "../spec_helper"
 
 describe CBOR::Lexer do
   describe "examples from the RFC7049 Appendix A" do
-    tests : Array(Tuple(String, Bytes)) = [
-      {"0", [0x00]},
-      {"1", [0x01]},
-      # {"10", "0a"},
-      # {"23", "17"},
-      # {"24", "18 18"},
-      # {"25", "18 19"},
-      # {"100", "18 64"},
-      # {"1000", "19 03 e8"},
-      # {"1000000", "1a 00 0f 42 40"},
-      # {"1000000000000", "1b 00 00 00 e8 d4 a5 10 00"},
-      # {"18446744073709551615", "1b ff ff ff ff ff ff ff ff"},
-      # {"18446744073709551616", "c2 49 01 00 00 00 00 00 00 0000"},
-      # {"-18446744073709551616", "3b ff ff ff ff ff ff ff ff"},
+    tests = [
+      {"0", "00"},
+      {"1", "01"},
+      {"10", "0a"},
+      {"23", "17"},
+      {"24", "18 18"},
+      {"25", "18 19"},
+      {"100", "18 64"},
+      {"1000", "19 03 e8"},
+      {"1000000", "1a 00 0f 42 40"},
+      {"1000000000000", "1b 00 00 00 e8 d4 a5 10 00"},
+      {"18446744073709551615", "1b ff ff ff ff ff ff ff ff"},
+      # {"18446744073709551616", "c2 49 01 00 00 00 00 00 00 00 00"},
+      {"-18446744073709551616", "3b ff ff ff ff ff ff ff ff"},
       # {"-18446744073709551617", "c3 49 01 00 00 00 00 00 00 00 00"},
-      # {"-1", "20"},
-      # {"-10", "29"},
-      # {"-100", "38 63"},
-      # {"-1000", "39 03 e7"},
+      {"-1", "20"},
+      {"-10", "29"},
+      {"-100", "38 63"},
+      {"-1000", "39 03 e7"},
     ]
 
     tests.each do |tt|
-      it "Reads #{tt[1].inspect} as #{tt[0]}" do
-        lexer = CBOR::Lexer.new(tt[1])
+      debug, hex = tt
+      it "Reads #{hex} as #{debug}" do
+        bytes = hex.split.map(&.to_u8(16))
+        lexer = CBOR::Lexer.new(Slice.new(bytes.to_unsafe, bytes.size))
 
         token = lexer.read_token
-        CBOR::Token.to_s(token).should eq(tt[0])
+        CBOR::Token.to_s(token).should eq(debug)
       end
     end
   end
diff --git a/spec/cbor_spec.cr b/spec/cbor_spec.cr
index 844aab6..4d6dc79 100644
--- a/spec/cbor_spec.cr
+++ b/spec/cbor_spec.cr
@@ -2,8 +2,4 @@ require "./spec_helper"
 
 describe CBOR do
   # TODO: Write tests
-
-  it "works" do
-    false.should eq(true)
-  end
 end
diff --git a/src/cbor/lexer.cr b/src/cbor/lexer.cr
index 8da4221..53261ff 100644
--- a/src/cbor/lexer.cr
+++ b/src/cbor/lexer.cr
@@ -51,27 +51,31 @@ class CBOR::Lexer
     when 0x00..0x17
       consume_int(current_byte)
     when 0x18
-      consume_int(read(Uint8))
+      consume_int(read(UInt8))
     when 0x19
-      consume_int(read(Uint16))
+      consume_int(read(UInt16))
     when 0x1a
-      consume_int(read(Uint32))
+      consume_int(read(UInt32))
     when 0x1b
-      consume_int(read(Uint64))
+      consume_int(read(UInt64))
     when 0x20..0x37
-      consume_int(flip(current_byte.to_i8))
+      # This range represents values from -1..-24 so we subtract 0x20
+      # from the uint8 value to
+      consume_int(to_negative_int(current_byte.to_u8 - 0x20))
     when 0x38
-      consume_int(flip(read(Uint8).to_i8))
+      consume_int(to_negative_int(read(UInt8)))
     when 0x39
-      consume_int(flip(read(Uint16).to_i16))
+      consume_int(to_negative_int(read(UInt16)))
     when 0x3a
-      consume_int(flip(read(Uint32).to_i32))
+      consume_int(to_negative_int(read(UInt32)))
     when 0x3b
-      consume_int(flip(read(Uint64).to_i64))
+      consume_int(to_negative_int(read(UInt64)))
+    else
+      fail
     end
   end
 
-  private def next_byte : Uint8
+  private def next_byte : UInt8
     byte = @io.read_byte
     @byte_number += 1
     fail unless byte
@@ -82,9 +86,24 @@ class CBOR::Lexer
     Token::IntT.new(@current_byte_number, value)
   end
 
-  private def flip(value)
-    -1 - value
-  end
+  {% begin %}
+    {% uints = %w(UInt8 UInt16 UInt32 UInt64) %}
+    {% conv = %w(to_i8 to_i16 to_i32 to_i64 to_i128) %}
+
+    {% for uint, index in uints %}
+      # Reads the `{{uint.id}}` as a negative integer, returning the samllest
+      # integer capable of containing the value.
+      def to_negative_int(value : {{uint.id}})
+        int = begin
+                -value.{{conv[index].id}}
+              rescue OverflowError
+                -value.{{conv[index + 1].id}}
+              end
+
+        int - 1
+      end
+    {% end %}
+  {% end %}
 
   private def read(type : T.class) forall T
     @byte_number += sizeof(T)
diff --git a/src/cbor/token.cr b/src/cbor/token.cr
index 118c840..6484e55 100644
--- a/src/cbor/token.cr
+++ b/src/cbor/token.cr
@@ -3,7 +3,7 @@ class CBOR::Token
   record BoolT, byte_number : Int32, value : Bool
   record ArrayT, byte_number : Int32, size : UInt32
   record MapT, byte_number : Int32, size : UInt32
-  record IntT, byte_number : Int32, value : Int8 | UInt8 | Int16 | UInt16 | Int32 | UInt32 | Int64 | UInt64
+  record IntT, byte_number : Int32, value : Int8 | UInt8 | Int16 | UInt16 | Int32 | UInt32 | Int64 | UInt64 | Int128
   record FloatT, byte_number : Int32, value : Float64
   record StringT, byte_number : Int32, value : String
   record BytesT, byte_number : Int32, value : Bytes