diff --git a/doc/specs/schemas/OpeningExplorerLichess.yaml b/doc/specs/schemas/OpeningExplorerLichess.yaml
index eb9e05f..0a90510 100644
--- a/doc/specs/schemas/OpeningExplorerLichess.yaml
+++ b/doc/specs/schemas/OpeningExplorerLichess.yaml
@@ -1,8 +1,9 @@
 type: object
 properties:
   opening:
-    $ref: './OpeningExplorerOpening.yaml'
-    nullable: true
+    oneOf:
+      - $ref: './OpeningExplorerOpening.yaml'
+      - type: 'null'
   white:
     type: number
   draws:
@@ -27,11 +28,13 @@ properties:
         black:
           type: number
         game:
-          $ref: './OpeningExplorerLichessGame.yaml'
-          nullable: true
+          oneOf:
+            - $ref: './OpeningExplorerLichessGame.yaml'
+            - type: 'null'
         opening:
-          $ref: './OpeningExplorerOpening.yaml'
-          nullable: true
+          oneOf:
+            - $ref: './OpeningExplorerOpening.yaml'
+            - type: 'null'
       required:
         - uci
         - san
diff --git a/doc/specs/schemas/OpeningExplorerLichessGame.yaml b/doc/specs/schemas/OpeningExplorerLichessGame.yaml
index 63fbc8b..759cc7f 100644
--- a/doc/specs/schemas/OpeningExplorerLichessGame.yaml
+++ b/doc/specs/schemas/OpeningExplorerLichessGame.yaml
@@ -3,11 +3,10 @@ properties:
   id:
     type: string
   winner:
-    type: string
+    type: [string, 'null']
     enum:
       - white
       - black
-    nullable: true
   speed:
     $ref: './Speed.yaml'
   white:
diff --git a/doc/specs/schemas/OpeningExplorerMasters.yaml b/doc/specs/schemas/OpeningExplorerMasters.yaml
index d9f7ceb..f138da1 100644
--- a/doc/specs/schemas/OpeningExplorerMasters.yaml
+++ b/doc/specs/schemas/OpeningExplorerMasters.yaml
@@ -1,8 +1,9 @@
 type: object
 properties:
   opening:
-    $ref: './OpeningExplorerOpening.yaml'
-    nullable: true
+    oneOf:
+      - $ref: './OpeningExplorerOpening.yaml'
+      - type: 'null'
   white:
     type: number
   draws:
@@ -26,11 +27,13 @@ properties:
         black:
           type: number
         game:
-          $ref: './OpeningExplorerMastersGame.yaml'
-          nullable: true
+          oneOf:
+            - $ref: './OpeningExplorerMastersGame.yaml'
+            - type: 'null'
         opening:
-          $ref: './OpeningExplorerOpening.yaml'
-          nullable: true
+          oneOf:
+            - $ref: './OpeningExplorerOpening.yaml'
+            - type: 'null'
       required:
         - uci
         - san
diff --git a/doc/specs/schemas/OpeningExplorerMastersGame.yaml b/doc/specs/schemas/OpeningExplorerMastersGame.yaml
index e77b1c0..297dafe 100644
--- a/doc/specs/schemas/OpeningExplorerMastersGame.yaml
+++ b/doc/specs/schemas/OpeningExplorerMastersGame.yaml
@@ -2,23 +2,22 @@ type: object
 properties:
   id:
     type: string
-  white:
-    $ref: './OpeningExplorerGamePlayer.yaml'
-  black:
-    $ref: './OpeningExplorerGamePlayer.yaml'
   winner:
-    type: string
+    type: [string, 'null']
     enum:
       - white
       - black
-    nullable: true
+  white:
+    $ref: './OpeningExplorerGamePlayer.yaml'
+  black:
+    $ref: './OpeningExplorerGamePlayer.yaml'
   year:
     type: number
   month:
     type: string
 required:
   - id
+  - winner
   - white
   - black
-  - winner
   - year
diff --git a/doc/specs/schemas/OpeningExplorerPlayer.yaml b/doc/specs/schemas/OpeningExplorerPlayer.yaml
index 3a3ab78..5f10278 100644
--- a/doc/specs/schemas/OpeningExplorerPlayer.yaml
+++ b/doc/specs/schemas/OpeningExplorerPlayer.yaml
@@ -1,8 +1,9 @@
 type: object
 properties:
   opening:
-    $ref: './OpeningExplorerOpening.yaml'
-    nullable: true
+    oneOf:
+      - $ref: './OpeningExplorerOpening.yaml'
+      - type: 'null'
   queuePosition:
     type: number
   white:
@@ -31,11 +32,13 @@ properties:
         black:
           type: number
         game:
-          $ref: './OpeningExplorerPlayerGame.yaml'
-          nullable: true
+          oneOf:
+            - $ref: './OpeningExplorerPlayerGame.yaml'
+            - type: 'null'
         opening:
-          $ref: './OpeningExplorerOpening.yaml'
-          nullable: true
+          oneOf:
+            - $ref: './OpeningExplorerOpening.yaml'
+            - type: 'null'
       required:
         - uci
         - san
diff --git a/doc/specs/schemas/OpeningExplorerPlayerGame.yaml b/doc/specs/schemas/OpeningExplorerPlayerGame.yaml
index 79910b0..218393f 100644
--- a/doc/specs/schemas/OpeningExplorerPlayerGame.yaml
+++ b/doc/specs/schemas/OpeningExplorerPlayerGame.yaml
@@ -3,11 +3,10 @@ properties:
   id:
     type: string
   winner:
-    type: string
+    type: [string, 'null']
     enum:
       - white
       - black
-    nullable: true
   speed:
     $ref: './Speed.yaml'
   mode: