diff --git a/src/big/json.cr b/src/big/json.cr
index 44d4ba6726dc..26682c6e3698 100644
--- a/src/big/json.cr
+++ b/src/big/json.cr
@@ -3,7 +3,7 @@ require "big"
 
 class JSON::Builder
   # Writes a big decimal.
-  def number(number : BigDecimal)
+  def number(number : BigDecimal) : Nil
     scalar do
       @io << number
     end
@@ -26,7 +26,7 @@ struct BigInt
     to_s
   end
 
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.number(self)
   end
 end
@@ -56,7 +56,7 @@ struct BigFloat
     to_s
   end
 
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.number(self)
   end
 end
@@ -86,7 +86,7 @@ struct BigDecimal
     to_s
   end
 
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.number(self)
   end
 end
diff --git a/src/compiler/crystal/tools/doc/type.cr b/src/compiler/crystal/tools/doc/type.cr
index faa09f51bf50..e435b52e74ce 100644
--- a/src/compiler/crystal/tools/doc/type.cr
+++ b/src/compiler/crystal/tools/doc/type.cr
@@ -786,7 +786,7 @@ class Crystal::Doc::Type
       builder.field "full_name", full_name
       builder.field "name", name
       builder.field "abstract", abstract?
-      builder.field "superclass" { superclass.try(&.to_json_simple(builder)) || builder.scalar(nil) }
+      builder.field "superclass" { (s = superclass) ? s.to_json_simple(builder) : builder.null }
       builder.field "ancestors" do
         builder.array do
           ancestors.each &.to_json_simple(builder)
@@ -821,7 +821,7 @@ class Crystal::Doc::Type
           including_types.each &.to_json_simple(builder)
         end
       end
-      builder.field "namespace" { namespace.try(&.to_json_simple(builder)) || builder.scalar(nil) }
+      builder.field "namespace" { (n = namespace) ? n.to_json_simple(builder) : builder.null }
       builder.field "doc", doc
       builder.field "summary", formatted_summary
       builder.field "class_methods", class_methods
diff --git a/src/json/builder.cr b/src/json/builder.cr
index 49af4b7bdb6d..65b10fc03c10 100644
--- a/src/json/builder.cr
+++ b/src/json/builder.cr
@@ -28,7 +28,7 @@ class JSON::Builder
   end
 
   # Starts a document.
-  def start_document
+  def start_document : Nil
     case state = @state.last
     when StartState
       @state[-1] = DocumentStartState.new
@@ -62,28 +62,28 @@ class JSON::Builder
   end
 
   # Writes a `null` value.
-  def null
+  def null : Nil
     scalar do
       @io << "null"
     end
   end
 
   # Writes a boolean value.
-  def bool(value : Bool)
+  def bool(value : Bool) : Nil
     scalar do
       @io << value
     end
   end
 
   # Writes an integer.
-  def number(number : Int)
+  def number(number : Int) : Nil
     scalar do
       @io << number
     end
   end
 
   # Writes a float.
-  def number(number : Float)
+  def number(number : Float) : Nil
     scalar do
       case number
       when .nan?
@@ -100,7 +100,7 @@ class JSON::Builder
   # by invoking `to_s` on it.
   #
   # This method can also be used to write the name of an object field.
-  def string(value)
+  def string(value) : Nil
     string = value.to_s
 
     scalar(string: true) do
@@ -157,14 +157,14 @@ class JSON::Builder
   # the IO without processing. This is the only method that
   # might lead to invalid JSON being generated, so you must
   # be sure that *string* contains a valid JSON string.
-  def raw(string : String)
+  def raw(string : String) : Nil
     scalar do
       @io << string
     end
   end
 
   # Writes the start of an array.
-  def start_array
+  def start_array : Nil
     start_scalar
     increase_indent
     @state.push ArrayState.new(empty: true)
@@ -172,7 +172,7 @@ class JSON::Builder
   end
 
   # Writes the end of an array.
-  def end_array
+  def end_array : Nil
     case state = @state.last
     when ArrayState
       @state.pop
@@ -193,7 +193,7 @@ class JSON::Builder
   end
 
   # Writes the start of an object.
-  def start_object
+  def start_object : Nil
     start_scalar
     increase_indent
     @state.push ObjectState.new(empty: true, name: true)
@@ -201,7 +201,7 @@ class JSON::Builder
   end
 
   # Writes the end of an object.
-  def end_object
+  def end_object : Nil
     case state = @state.last
     when ObjectState
       unless state.name
@@ -235,12 +235,12 @@ class JSON::Builder
   end
 
   # :ditto:
-  def scalar(value : Int | Float)
+  def scalar(value : Int | Float) : Nil
     number(value)
   end
 
   # :ditto:
-  def scalar(value : String)
+  def scalar(value : String) : Nil
     string(value)
   end
 
diff --git a/src/json/pull_parser.cr b/src/json/pull_parser.cr
index 2f3758d99d3f..7258d6d958f0 100644
--- a/src/json/pull_parser.cr
+++ b/src/json/pull_parser.cr
@@ -265,7 +265,7 @@ class JSON::PullParser
   # Reads the new value and fill the a JSON builder with it.
   #
   # Use this method with a `JSON::Builder` to read a JSON while building another one.
-  def read_raw(json)
+  def read_raw(json) : Nil
     case @kind
     when .null?
       read_next
@@ -558,7 +558,7 @@ class JSON::PullParser
   #
   # It skips the whole value, not only the next lexer's token.
   # For example if the next value is an array, the whole array will be skipped.
-  def skip
+  def skip : Nil
     @lexer.skip = true
     skip_internal
     @lexer.skip = false
diff --git a/src/json/to_json.cr b/src/json/to_json.cr
index 0c0bcf482367..2b3224529e84 100644
--- a/src/json/to_json.cr
+++ b/src/json/to_json.cr
@@ -25,7 +25,7 @@ class Object
 end
 
 struct Nil
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.null
   end
 
@@ -35,13 +35,13 @@ struct Nil
 end
 
 struct Bool
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.bool(self)
   end
 end
 
 struct Int
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.number(self)
   end
 
@@ -51,7 +51,7 @@ struct Int
 end
 
 struct Float
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.number(self)
   end
 
@@ -61,7 +61,7 @@ struct Float
 end
 
 class String
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.string(self)
   end
 
@@ -71,7 +71,7 @@ class String
 end
 
 struct Path
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     @name.to_json(json)
   end
 
@@ -81,7 +81,7 @@ struct Path
 end
 
 struct Symbol
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.string(to_s)
   end
 
@@ -91,7 +91,7 @@ struct Symbol
 end
 
 class Array
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.array do
       each &.to_json(json)
     end
@@ -99,7 +99,7 @@ class Array
 end
 
 class Deque
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.array do
       each &.to_json(json)
     end
@@ -107,7 +107,7 @@ class Deque
 end
 
 struct Set
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.array do
       each &.to_json(json)
     end
@@ -120,7 +120,7 @@ class Hash
   # Keys are serialized by invoking `to_json_object_key` on them.
   # Values are serialized with the usual `to_json(json : JSON::Builder)`
   # method.
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.object do
       each do |key, value|
         json.field key.to_json_object_key do
@@ -132,7 +132,7 @@ class Hash
 end
 
 struct Tuple
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.array do
       {% for i in 0...T.size %}
         self[{{i}}].to_json(json)
@@ -154,7 +154,7 @@ struct NamedTuple
 end
 
 struct Time::Format
-  def to_json(value : Time, json : JSON::Builder)
+  def to_json(value : Time, json : JSON::Builder) : Nil
     format(value).to_json(json)
   end
 end
@@ -263,7 +263,7 @@ struct Time
   # a time value.
   #
   # See `#from_json` for reference.
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.string(Time::Format::RFC_3339.format(self, fraction_digits: 0))
   end
 end
@@ -343,7 +343,7 @@ end
 # person.to_json    # => %({"birth_date":1459859781})
 # ```
 module Time::EpochConverter
-  def self.to_json(value : Time, json : JSON::Builder)
+  def self.to_json(value : Time, json : JSON::Builder) : Nil
     json.number(value.to_unix)
   end
 end
@@ -367,7 +367,7 @@ end
 # timestamp.to_json # => %({"value":1459860483856})
 # ```
 module Time::EpochMillisConverter
-  def self.to_json(value : Time, json : JSON::Builder)
+  def self.to_json(value : Time, json : JSON::Builder) : Nil
     json.number(value.to_unix_ms)
   end
 end
@@ -394,7 +394,7 @@ end
 # raw.to_json # => %({"value":123456789876543212345678987654321})
 # ```
 module String::RawConverter
-  def self.to_json(value : String, json : JSON::Builder)
+  def self.to_json(value : String, json : JSON::Builder) : Nil
     json.raw(value)
   end
 end
diff --git a/src/oauth2/access_token/bearer.cr b/src/oauth2/access_token/bearer.cr
index 9e5a5c0148d3..c586dbb2cd47 100644
--- a/src/oauth2/access_token/bearer.cr
+++ b/src/oauth2/access_token/bearer.cr
@@ -13,7 +13,7 @@ class OAuth2::AccessToken::Bearer < OAuth2::AccessToken
     request.headers["Authorization"] = "Bearer #{access_token}"
   end
 
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.object do
       json.field "token_type", "bearer"
       json.field "access_token", access_token
diff --git a/src/oauth2/access_token/mac.cr b/src/oauth2/access_token/mac.cr
index 670b57357662..6f7a8f44cbf4 100644
--- a/src/oauth2/access_token/mac.cr
+++ b/src/oauth2/access_token/mac.cr
@@ -45,7 +45,7 @@ class OAuth2::AccessToken::Mac < OAuth2::AccessToken
     Base64.strict_encode OpenSSL::HMAC.digest(digest, mac_key, normalized_request_string)
   end
 
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.object do
       json.field "token_type", "mac"
       json.field "access_token", access_token
diff --git a/src/uuid/json.cr b/src/uuid/json.cr
index d9b56f7258ca..040668f4a6b8 100644
--- a/src/uuid/json.cr
+++ b/src/uuid/json.cr
@@ -32,7 +32,7 @@ struct UUID
   # uuid = UUID.new("87b3042b-9b9a-41b7-8b15-a93d3f17025e")
   # uuid.to_json # => "\"87b3042b-9b9a-41b7-8b15-a93d3f17025e\""
   # ```
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     json.string(to_s)
   end
 
diff --git a/src/yaml/any.cr b/src/yaml/any.cr
index 880f0c75c330..71b6b2e2e379 100644
--- a/src/yaml/any.cr
+++ b/src/yaml/any.cr
@@ -312,7 +312,7 @@ struct YAML::Any
     raw.to_yaml(io)
   end
 
-  def to_json(builder : JSON::Builder)
+  def to_json(builder : JSON::Builder) : Nil
     if (raw = self.raw).is_a?(Slice)
       raise "Can't serialize #{raw.class} to JSON"
     else
diff --git a/src/yaml/builder.cr b/src/yaml/builder.cr
index 1e96dd5142a8..73ec669c25cf 100644
--- a/src/yaml/builder.cr
+++ b/src/yaml/builder.cr
@@ -61,7 +61,7 @@ class YAML::Builder
   end
 
   # Ends a YAML stream.
-  def end_stream
+  def end_stream : Nil
     emit stream_end
     flush
   end
@@ -96,14 +96,14 @@ class YAML::Builder
   end
 
   # Starts a sequence.
-  def start_sequence(anchor : String? = nil, tag : String? = nil, style : YAML::SequenceStyle = YAML::SequenceStyle::ANY)
+  def start_sequence(anchor : String? = nil, tag : String? = nil, style : YAML::SequenceStyle = YAML::SequenceStyle::ANY) : Nil
     implicit = tag ? 0 : 1
     emit sequence_start, get_anchor(anchor), string_to_unsafe(tag), implicit, style
     increase_nesting
   end
 
   # Ends a sequence.
-  def end_sequence
+  def end_sequence : Nil
     emit sequence_end
     decrease_nesting
   end
@@ -115,14 +115,14 @@ class YAML::Builder
   end
 
   # Starts a mapping.
-  def start_mapping(anchor : String? = nil, tag : String? = nil, style : YAML::MappingStyle = YAML::MappingStyle::ANY)
+  def start_mapping(anchor : String? = nil, tag : String? = nil, style : YAML::MappingStyle = YAML::MappingStyle::ANY) : Nil
     implicit = tag ? 0 : 1
     emit mapping_start, get_anchor(anchor), string_to_unsafe(tag), implicit, style
     increase_nesting
   end
 
   # Ends a mapping.
-  def end_mapping
+  def end_mapping : Nil
     emit mapping_end
     decrease_nesting
   end
@@ -185,7 +185,7 @@ class YAML::Builder
   end
 
   # Closes the builder, freeing up resources.
-  def close
+  def close : Nil
     finalize
     @closed = true
   end
diff --git a/src/yaml/nodes/parser.cr b/src/yaml/nodes/parser.cr
index 20fd0aed550b..d63cf093196c 100644
--- a/src/yaml/nodes/parser.cr
+++ b/src/yaml/nodes/parser.cr
@@ -46,7 +46,7 @@ class YAML::Nodes::Parser < YAML::Parser
     node.start_column = @pull_parser.start_column.to_i
   end
 
-  def end_value(node)
+  def end_value(node) : Nil
     node.end_line = @pull_parser.end_line.to_i
     node.end_column = @pull_parser.end_column.to_i
   end
@@ -72,15 +72,15 @@ class YAML::Nodes::Parser < YAML::Parser
     documents << document
   end
 
-  def add_to_document(document, node)
+  def add_to_document(document, node) : Nil
     document << node
   end
 
-  def add_to_sequence(sequence, node)
+  def add_to_sequence(sequence, node) : Nil
     sequence << node
   end
 
-  def add_to_mapping(mapping, key, value)
+  def add_to_mapping(mapping, key, value) : Nil
     mapping[key] = value
   end
 end
diff --git a/src/yaml/parser.cr b/src/yaml/parser.cr
index 547ff56a72e8..5b27cee2a3b9 100644
--- a/src/yaml/parser.cr
+++ b/src/yaml/parser.cr
@@ -157,7 +157,7 @@ abstract class YAML::Parser
   end
 
   # Closes this parser, freeing up resources.
-  def close
+  def close : Nil
     @pull_parser.close
   end
 
diff --git a/src/yaml/pull_parser.cr b/src/yaml/pull_parser.cr
index f24a8a5182a8..73742cb46e07 100644
--- a/src/yaml/pull_parser.cr
+++ b/src/yaml/pull_parser.cr
@@ -321,7 +321,7 @@ class YAML::PullParser
     LibYAML.yaml_event_delete(pointerof(@event))
   end
 
-  def close
+  def close : Nil
     finalize
     @closed = true
   end
diff --git a/src/yaml/schema/core/parser.cr b/src/yaml/schema/core/parser.cr
index 0b4402b853a7..bc17ef63856d 100644
--- a/src/yaml/schema/core/parser.cr
+++ b/src/yaml/schema/core/parser.cr
@@ -36,15 +36,15 @@ class YAML::Schema::Core::Parser < YAML::Parser
     Any.new(Core.parse_scalar(@pull_parser))
   end
 
-  def add_to_documents(documents, document)
+  def add_to_documents(documents, document) : Nil
     documents << document
   end
 
-  def add_to_document(document, node)
+  def add_to_document(document, node) : Nil
     document.as_a << node
   end
 
-  def add_to_sequence(sequence, node)
+  def add_to_sequence(sequence, node) : Nil
     sequence.as_a << node
   end
 
diff --git a/src/yaml/schema/fail_safe.cr b/src/yaml/schema/fail_safe.cr
index f309537f67b8..2518f104c447 100644
--- a/src/yaml/schema/fail_safe.cr
+++ b/src/yaml/schema/fail_safe.cr
@@ -50,19 +50,19 @@ module YAML::Schema::FailSafe
       Any.new(@pull_parser.value)
     end
 
-    def add_to_documents(documents, document)
+    def add_to_documents(documents, document) : Nil
       documents << document
     end
 
-    def add_to_document(document, node)
+    def add_to_document(document, node) : Nil
       document.as_a << node
     end
 
-    def add_to_sequence(sequence, node)
+    def add_to_sequence(sequence, node) : Nil
       sequence.as_a << node
     end
 
-    def add_to_mapping(mapping, key, value)
+    def add_to_mapping(mapping, key, value) : Nil
       mapping.as_h[key] = value
     end
   end