diff --git a/README.md b/README.md
index dbd01a4..081f740 100644
--- a/README.md
+++ b/README.md
@@ -35,7 +35,7 @@ Please see [mal test cases](https://github.com/rhysd/Crisp/tree/master/spec/cris
 ## Development Environment
 
 - OS X
-- Crystal 0.7.1 ~ 0.7.3 or 0.7.7 ~ 0.11.0
+- Crystal 0.7.1 ~ 0.7.3 or 0.7.7 ~ 0.11.0 or 0.17.4
 
 ## License
 
diff --git a/src/crisp/core.cr b/src/crisp/core.cr
index fe2b148..e21b626 100644
--- a/src/crisp/core.cr
+++ b/src/crisp/core.cr
@@ -149,7 +149,7 @@ module Crisp
     end
   end
 
-  def nil?(args)
+  def nil_value?(args)
     args.first.unwrap.nil?
   end
 
@@ -380,7 +380,7 @@ module Crisp
     "throw"       => -> (args : Array(Crisp::Expr)) { raise Crisp::RuntimeException.new args[0] },
     "apply"       => func(:apply),
     "map"         => func(:map),
-    "nil?"        => func(:nil?),
+    "nil?"        => func(:nil_value?),
     "true?"       => func(:true?),
     "false?"      => func(:false?),
     "symbol?"     => func(:symbol?),
diff --git a/src/crisp/env.cr b/src/crisp/env.cr
index 1dea91c..3cc6306 100644
--- a/src/crisp/env.cr
+++ b/src/crisp/env.cr
@@ -6,7 +6,7 @@ module Crisp
   class Env
     property data
 
-    def initialize(@outer = nil)
+    def initialize(@outer : Env? = nil)
       @data = {} of String => Crisp::Expr
     end
 
diff --git a/src/crisp/error.cr b/src/crisp/error.cr
index c715676..d24bda1 100644
--- a/src/crisp/error.cr
+++ b/src/crisp/error.cr
@@ -9,7 +9,7 @@ module Crisp
 
   class RuntimeException < Exception
     getter :thrown
-    def initialize(@thrown)
+    def initialize(@thrown : Crisp::Expr)
       super()
     end
   end
diff --git a/src/crisp/expr.cr b/src/crisp/expr.cr
index 0b6ab54..eccf59b 100644
--- a/src/crisp/expr.cr
+++ b/src/crisp/expr.cr
@@ -5,7 +5,7 @@ module Crisp
 
   class Symbol
     property :str
-    def initialize(@str)
+    def initialize(@str : String)
     end
 
     def ==(other : Symbol)
@@ -24,7 +24,7 @@ module Crisp
 
   class Atom
     property :val
-    def initialize(@val)
+    def initialize(@val : Expr)
     end
 
     def ==(rhs : Atom)
@@ -34,7 +34,7 @@ module Crisp
 
   class Closure
     property :ast, :params, :env, :fn
-    def initialize(@ast, @params, @env, @fn)
+    def initialize(@ast : Expr, @params : List | Vector, @env : Env, @fn : Func)
     end
   end
 
@@ -42,14 +42,11 @@ module Crisp
     alias Func = (Array(Expr) -> Expr)
     alias Type = Nil | Bool | Int32 | String | Symbol | List | Vector | HashMap | Func | Closure | Atom
 
-    is_macro :: Bool
-    meta :: Expr
-
     property :is_macro, :meta
 
     def initialize(@val : Type)
       @is_macro = false
-      @meta = nil
+      @meta = nil as Expr?
     end
 
     def initialize(other : Expr)
diff --git a/src/crisp/reader.cr b/src/crisp/reader.cr
index 7efc5c5..224647e 100644
--- a/src/crisp/reader.cr
+++ b/src/crisp/reader.cr
@@ -5,7 +5,7 @@ module Crisp
   extend self
 
   class Reader
-    def initialize(@tokens)
+    def initialize(@tokens : Array(String))
       @pos = 0
     end