diff --git a/codegen/lib/graphql_swift_gen/templates/type.swift.erb b/codegen/lib/graphql_swift_gen/templates/type.swift.erb
index 1a2965a..5bc3c43 100644
--- a/codegen/lib/graphql_swift_gen/templates/type.swift.erb
+++ b/codegen/lib/graphql_swift_gen/templates/type.swift.erb
@@ -210,33 +210,54 @@ extension <%= schema_name %> {
     }
   <% when 'INPUT_OBJECT' %>
     open class <%= type.name %> {
-      <% type.input_fields.each do |field| %>
+      <% type.required_input_fields.each do |field| %>
        open var <%= escape_reserved_word(field.camelize_name) %>: <%= swift_input_type(field.type) %>
       <% end %>
+      <% type.optional_input_fields.each do |field| %>
+        open var <%= escape_reserved_word(field.camelize_name) %>: <%= swift_input_type(field.type) %> {
+          didSet {
+            <%= escape_reserved_word(field.camelize_name) %>Seen = true
+          }
+        }
+        private var <%= escape_reserved_word(field.camelize_name) %>Seen: Bool = false
+      <% end %>
 
       public init(
-        <% input_fields = type.required_input_fields + type.optional_input_fields %>
+        <% input_fields = type.required_input_fields + type.optional_input_fields # manually ordered with required fields first %>
         <% input_fields.each do |field| %>
-          <% default = field.type.non_null? ? "" : " = nil" %>
           <% seperator = field == input_fields.last ? "" : "," %>
-          <%= escape_reserved_word(field.camelize_name) %>: <%= swift_input_type(field.type) %><%= default %><%= seperator %>
+          <% if field.type.non_null? %>
+            <%= escape_reserved_word(field.camelize_name) %>: <%= swift_input_type(field.type) %><%= seperator %>
+          <% else %>
+            <%= escape_reserved_word(field.camelize_name) %>: <%= swift_input_type(field.type) %>? = nil<%= seperator %>
+          <% end %>
         <% end %>
       ) {
-        <% type.input_fields.each do |field| %>
+        <% type.required_input_fields.each do |field| %>
           self.<%= escape_reserved_word(field.camelize_name) %> = <%= escape_reserved_word(field.camelize_name) %>
         <% end %>
+        <% type.optional_input_fields.each do |field| %>
+          <% field_name = escape_reserved_word(field.camelize_name) %>
+          if let <%= field_name %> = <%= field_name %> {
+            self.<%= field_name %>Seen = true
+            self.<%= field_name %> = <%= field_name %>
+          }
+        <% end %>
       }
 
       func serialize() -> String {
         var fields: [String] = []
-        <% type.input_fields.each do |field| %>
-          <% unless field.type.non_null? %>
+        <% type.required_input_fields.each do |field| %>
+          fields.append("<%= field.name %>:<%= generate_build_input_code(field.camelize_name, field.type.unwrap_non_null) %>")
+        <% end %>
+        <% type.optional_input_fields.each do |field| %>
+          if <%= escape_reserved_word(field.camelize_name) %>Seen {
             if let <%= escape_reserved_word(field.camelize_name) %> = <%= escape_reserved_word(field.camelize_name) %> {
-          <% end %>
               fields.append("<%= field.name %>:<%= generate_build_input_code(field.camelize_name, field.type.unwrap_non_null) %>")
-          <% unless field.type.non_null? %>
+            } else {
+              fields.append("<%= field.name %>:null")
             }
-          <% end %>
+          }
         <% end %>
         return "{\(fields.joined(separator: ","))}"
       }
diff --git a/support/Tests/GraphQLSupportTests/IntegrationTests.swift b/support/Tests/GraphQLSupportTests/IntegrationTests.swift
index 0824aaf..852d8e4 100644
--- a/support/Tests/GraphQLSupportTests/IntegrationTests.swift
+++ b/support/Tests/GraphQLSupportTests/IntegrationTests.swift
@@ -56,6 +56,24 @@ class IntegrationTests: XCTestCase {
         XCTAssertEqual(queryString, "mutation{set_integer(input:{key:\"answer\",value:42,negate:true})}")
     }
 
+    func testInputObjectExplictNilConstructor() {
+        let query = Generated.buildMutation { $0
+            .setInteger(input: Generated.SetIntegerInput(key: "answer", value: 42, negate: .some(nil)))
+        }
+        let queryString = String(describing: query)
+        XCTAssertEqual(queryString, "mutation{set_integer(input:{key:\"answer\",value:42,negate:null})}")
+    }
+
+    func testInputObjectExplictNilSetLater() {
+        let input = Generated.SetIntegerInput(key: "answer", value: 42)
+        input.negate = nil
+        let query = Generated.buildMutation { $0
+            .setInteger(input: input)
+        }
+        let queryString = String(describing: query)
+        XCTAssertEqual(queryString, "mutation{set_integer(input:{key:\"answer\",value:42,negate:null})}")
+    }
+
     func testScalarInput() {
         let ttl = date(year: 2017, month: 1, day: 31, hour: 10, minute: 9, second: 48)
         let query = Generated.buildMutation { $0