diff --git a/lib/fetch/form_data.rb b/lib/fetch/form_data.rb
index 1c250b3..f1f1c78 100644
--- a/lib/fetch/form_data.rb
+++ b/lib/fetch/form_data.rb
@@ -25,7 +25,7 @@ def initialize
     attr_reader :entries
 
     def append(key, value)
-      @entries.push [key.to_s, value.is_a?(File) ? value : value.to_s]
+      @entries.push [key.to_s, File === value ? value : value.to_s]
     end
 
     def delete(key)
diff --git a/sig/fetch.rbs b/sig/fetch.rbs
index 8ae6c95..dfd4e5d 100644
--- a/sig/fetch.rbs
+++ b/sig/fetch.rbs
@@ -1,7 +1,3 @@
 module Fetch
-  interface _StringLike
-    def to_str: () -> String
-  end
-
   VERSION: String
 end
diff --git a/sig/fetch/_client.rbs b/sig/fetch/_client.rbs
index e844807..05183ab 100644
--- a/sig/fetch/_client.rbs
+++ b/sig/fetch/_client.rbs
@@ -1,5 +1,5 @@
 module Fetch
   class Client
-    def to_response: (_StringLike, Net::HTTPResponse, bool) -> Response
+    def to_response: (string, Net::HTTPResponse, bool) -> Response
   end
 end
diff --git a/sig/fetch/api.rbs b/sig/fetch/api.rbs
index f7136b3..cb93cd3 100644
--- a/sig/fetch/api.rbs
+++ b/sig/fetch/api.rbs
@@ -1,9 +1,9 @@
 module Fetch
   class API
     def self?.fetch: (
-      _StringLike,
+      string,
       ?method: String | Symbol,
-      ?headers: Enumerable[[Object, Object]],
+      ?headers: _Each[[_ToS, _ToS]],
       ?body: (String | FormData | URLSearchParams)?,
       ?redirect: 'follow' | 'error' | 'manual' | :follow | :error | :manual,
     ) -> Response
diff --git a/sig/fetch/client.rbs b/sig/fetch/client.rbs
index 5eb782b..a4e9224 100644
--- a/sig/fetch/client.rbs
+++ b/sig/fetch/client.rbs
@@ -3,9 +3,9 @@ module Fetch
     include Singleton
 
     def fetch: (
-      _StringLike,
+      string,
       ?method: String | Symbol,
-      ?headers: Enumerable[[Object, Object]],
+      ?headers: _Each[[_ToS, _ToS]],
       ?body: (String | FormData | URLSearchParams)?,
       ?redirect: 'follow' | 'error' | 'manual' | :follow | :error | :manual,
       ?_redirected: bool
diff --git a/sig/fetch/form_data.rbs b/sig/fetch/form_data.rbs
index e763c51..1a06984 100644
--- a/sig/fetch/form_data.rbs
+++ b/sig/fetch/form_data.rbs
@@ -1,24 +1,24 @@
 module Fetch
   class FormData
-    type value = String | File
-
-    include Enumerable[[String, value]]
+    include Enumerable[[String, String | File]]
 
     extend Forwardable
 
-    attr_reader entries: Array[[String, value]]
+    attr_reader entries: Array[[String, String | File]]
 
-    def self.build: (Enumerable[[Object, Object]]) -> FormData
+    def self.build: (_Each[[_ToS, _ToS | File]]) -> FormData
 
     def initialize: () -> void
-    def append: (Object, Object) -> void
-    def delete: (Object) -> void
-    def get: (Object) -> value?
-    def get_all: (Object) -> Array[value]
-    def has: (Object) -> bool
+    def append: (_ToS, _ToS | File) -> void
+    def delete: (_ToS) -> void
+    def get: (_ToS) -> (String | File)?
+    def get_all: (_ToS) -> Array[String | File]
+    def has: (_ToS) -> bool
     def keys: -> Array[String]
-    def set: (Object, Object) -> void
-    def values: () -> Array[value]
-    def each: () { ([String, value]) -> void } -> Array[[String, value]]
+    def set: (_ToS, _ToS | File) -> void
+    def values: () -> Array[String | File]
+
+    def each: () { ([String, String | File]) -> void } -> Array[[String, String | File]]
+            | () -> Enumerator[[String, String | File], Array[[String, String | File]]]
   end
 end
diff --git a/sig/fetch/headers.rbs b/sig/fetch/headers.rbs
index 9238c25..1770d46 100644
--- a/sig/fetch/headers.rbs
+++ b/sig/fetch/headers.rbs
@@ -4,15 +4,17 @@ module Fetch
 
     extend Forwardable
 
-    def initialize: (Enumerable[[Object, Object]]) -> void
-    def append: (Object, Object) -> void
-    def delete: (Object) -> void
+    def initialize: (_Each[[_ToS, _ToS]]) -> void
+    def append: (_ToS, _ToS) -> void
+    def delete: (_ToS) -> void
     def entries: () -> Array[[String, String]]
-    def get: (Object) -> String?
-    def has: (Object) -> bool
+    def get: (_ToS) -> String?
+    def has: (_ToS) -> bool
     def keys: -> Array[String]
-    def set: (Object, Object) -> void
+    def set: (_ToS, _ToS) -> void
     def values: () -> Array[String]
+
     def each: () { ([String, String]) -> void } -> Array[[String, String]]
+            | () -> Enumerator[[String, String], Array[[String, String]]]
   end
 end
diff --git a/sig/fetch/url_search_params.rbs b/sig/fetch/url_search_params.rbs
index 0de936e..24f5775 100644
--- a/sig/fetch/url_search_params.rbs
+++ b/sig/fetch/url_search_params.rbs
@@ -6,17 +6,19 @@ module Fetch
 
     attr_reader entries: Array[[String, String]]
 
-    def self.build: (Enumerable[[Object, Object]]) -> FormData
+    def self.build: (_Each[[_ToS, _ToS]]) -> FormData
 
-    def initialize: (String | Enumerable[[Object, Object]]) -> void
-    def append: (Object, Object) -> void
-    def delete: (Object) -> void
-    def get: (Object) -> String?
-    def get_all: (Object) -> Array[String]
-    def has: (Object) -> bool
+    def initialize: (String | _Each[[_ToS, _ToS]]) -> void
+    def append: (_ToS, _ToS) -> void
+    def delete: (_ToS) -> void
+    def get: (_ToS) -> String?
+    def get_all: (_ToS) -> Array[String]
+    def has: (_ToS) -> bool
     def keys: -> Array[String]
-    def set: (Object, Object) -> void
+    def set: (_ToS, _ToS) -> void
     def values: () -> Array[String]
+
     def each: () { ([String, String]) -> void } -> Array[[String, String]]
+            | () -> Enumerator[[String, String], Array[[String, String]]]
   end
 end