diff --git a/src/type/definition.js b/src/type/definition.js
index 6f1b737498..739732cd5f 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -607,7 +607,7 @@ export class GraphQLScalarType {
     parseValue: GraphQLScalarValueParser<mixed>,
     parseLiteral: GraphQLScalarLiteralParser<mixed>,
     extensions: ?ReadOnlyObjMap<mixed>,
-    extensionASTNodes: ?$ReadOnlyArray<ScalarTypeExtensionNode>,
+    extensionASTNodes: $ReadOnlyArray<ScalarTypeExtensionNode>,
   |} {
     return {
       name: this.name,
@@ -617,7 +617,7 @@ export class GraphQLScalarType {
       parseLiteral: this.parseLiteral,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes,
+      extensionASTNodes: this.extensionASTNodes ?? [],
     };
   }
 
@@ -745,7 +745,7 @@ export class GraphQLObjectType {
     interfaces: Array<GraphQLInterfaceType>,
     fields: GraphQLFieldConfigMap<any, any>,
     extensions: ?ReadOnlyObjMap<mixed>,
-    extensionASTNodes: ?$ReadOnlyArray<ObjectTypeExtensionNode>,
+    extensionASTNodes: $ReadOnlyArray<ObjectTypeExtensionNode>,
   |} {
     return {
       name: this.name,
@@ -755,7 +755,7 @@ export class GraphQLObjectType {
       isTypeOf: this.isTypeOf,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes,
+      extensionASTNodes: this.extensionASTNodes || [],
     };
   }
 
@@ -1055,7 +1055,7 @@ export class GraphQLInterfaceType {
     interfaces: Array<GraphQLInterfaceType>,
     fields: GraphQLFieldConfigMap<any, any>,
     extensions: ?ReadOnlyObjMap<mixed>,
-    extensionASTNodes: ?$ReadOnlyArray<InterfaceTypeExtensionNode>,
+    extensionASTNodes: $ReadOnlyArray<InterfaceTypeExtensionNode>,
   |} {
     return {
       name: this.name,
@@ -1065,7 +1065,7 @@ export class GraphQLInterfaceType {
       resolveType: this.resolveType,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes,
+      extensionASTNodes: this.extensionASTNodes ?? [],
     };
   }
 
@@ -1158,7 +1158,7 @@ export class GraphQLUnionType {
     ...GraphQLUnionTypeConfig<any, any>,
     types: Array<GraphQLObjectType>,
     extensions: ?ReadOnlyObjMap<mixed>,
-    extensionASTNodes: ?$ReadOnlyArray<UnionTypeExtensionNode>,
+    extensionASTNodes: $ReadOnlyArray<UnionTypeExtensionNode>,
   |} {
     return {
       name: this.name,
@@ -1167,7 +1167,7 @@ export class GraphQLUnionType {
       resolveType: this.resolveType,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes,
+      extensionASTNodes: this.extensionASTNodes ?? [],
     };
   }
 
@@ -1320,7 +1320,7 @@ export class GraphQLEnumType /* <T> */ {
   toConfig(): {|
     ...GraphQLEnumTypeConfig,
     extensions: ?ReadOnlyObjMap<mixed>,
-    extensionASTNodes: ?$ReadOnlyArray<EnumTypeExtensionNode>,
+    extensionASTNodes: $ReadOnlyArray<EnumTypeExtensionNode>,
   |} {
     const values = keyValMap(
       this.getValues(),
@@ -1340,7 +1340,7 @@ export class GraphQLEnumType /* <T> */ {
       values,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes,
+      extensionASTNodes: this.extensionASTNodes ?? [],
     };
   }
 
@@ -1476,7 +1476,7 @@ export class GraphQLInputObjectType {
     ...GraphQLInputObjectTypeConfig,
     fields: GraphQLInputFieldConfigMap,
     extensions: ?ReadOnlyObjMap<mixed>,
-    extensionASTNodes: ?$ReadOnlyArray<InputObjectTypeExtensionNode>,
+    extensionASTNodes: $ReadOnlyArray<InputObjectTypeExtensionNode>,
   |} {
     const fields = mapValue(this.getFields(), field => ({
       description: field.description,
@@ -1492,7 +1492,7 @@ export class GraphQLInputObjectType {
       fields,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes,
+      extensionASTNodes: this.extensionASTNodes ?? [],
     };
   }
 
diff --git a/src/type/schema.js b/src/type/schema.js
index 316b1c0b99..5cb5241547 100644
--- a/src/type/schema.js
+++ b/src/type/schema.js
@@ -345,7 +345,7 @@ export class GraphQLSchema {
       directives: this.getDirectives().slice(),
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes,
+      extensionASTNodes: this.extensionASTNodes ?? [],
       assumeValid: this.__validationErrors !== undefined,
     };
   }
@@ -391,7 +391,7 @@ export type GraphQLSchemaNormalizedConfig = {|
   types: Array<GraphQLNamedType>,
   directives: Array<GraphQLDirective>,
   extensions: ?ReadOnlyObjMap<mixed>,
-  extensionASTNodes: ?$ReadOnlyArray<SchemaExtensionNode>,
+  extensionASTNodes: $ReadOnlyArray<SchemaExtensionNode>,
   assumeValid: boolean,
 |};
 
diff --git a/src/utilities/extendSchema.js b/src/utilities/extendSchema.js
index a2b7202ea1..e76041c65e 100644
--- a/src/utilities/extendSchema.js
+++ b/src/utilities/extendSchema.js
@@ -228,10 +228,7 @@ export function extendSchemaImpl(
     ],
     extensions: undefined,
     astNode: schemaDef ?? schemaConfig.astNode,
-    extensionASTNodes: concatMaybeArrays(
-      schemaConfig.extensionASTNodes,
-      schemaExtensions,
-    ),
+    extensionASTNodes: schemaConfig.extensionASTNodes.concat(schemaExtensions),
     assumeValid: options?.assumeValid ?? false,
   };
 
@@ -305,10 +302,7 @@ export function extendSchemaImpl(
         })),
         ...buildInputFieldMap(extensions),
       }),
-      extensionASTNodes: concatMaybeArrays(
-        config.extensionASTNodes,
-        extensions,
-      ),
+      extensionASTNodes: config.extensionASTNodes.concat(extensions),
     });
   }
 
@@ -322,10 +316,7 @@ export function extendSchemaImpl(
         ...config.values,
         ...buildEnumValueMap(extensions),
       },
-      extensionASTNodes: concatMaybeArrays(
-        config.extensionASTNodes,
-        extensions,
-      ),
+      extensionASTNodes: config.extensionASTNodes.concat(extensions),
     });
   }
 
@@ -335,10 +326,7 @@ export function extendSchemaImpl(
 
     return new GraphQLScalarType({
       ...config,
-      extensionASTNodes: concatMaybeArrays(
-        config.extensionASTNodes,
-        extensions,
-      ),
+      extensionASTNodes: config.extensionASTNodes.concat(extensions),
     });
   }
 
@@ -356,10 +344,7 @@ export function extendSchemaImpl(
         ...mapValue(config.fields, extendField),
         ...buildFieldMap(extensions),
       }),
-      extensionASTNodes: concatMaybeArrays(
-        config.extensionASTNodes,
-        extensions,
-      ),
+      extensionASTNodes: config.extensionASTNodes.concat(extensions),
     });
   }
 
@@ -379,10 +364,7 @@ export function extendSchemaImpl(
         ...mapValue(config.fields, extendField),
         ...buildFieldMap(extensions),
       }),
-      extensionASTNodes: concatMaybeArrays(
-        config.extensionASTNodes,
-        extensions,
-      ),
+      extensionASTNodes: config.extensionASTNodes.concat(extensions),
     });
   }
 
@@ -396,10 +378,7 @@ export function extendSchemaImpl(
         ...type.getTypes().map(replaceNamedType),
         ...buildUnionTypes(extensions),
       ],
-      extensionASTNodes: concatMaybeArrays(
-        config.extensionASTNodes,
-        extensions,
-      ),
+      extensionASTNodes: config.extensionASTNodes.concat(extensions),
     });
   }
 
@@ -704,19 +683,6 @@ export function extendSchemaImpl(
   }
 }
 
-function concatMaybeArrays<X>(
-  maybeArrayA: ?$ReadOnlyArray<X>,
-  maybeArrayB: ?$ReadOnlyArray<X>,
-): ?$ReadOnlyArray<X> {
-  if (maybeArrayA == null) {
-    return maybeArrayB;
-  }
-  if (maybeArrayB == null) {
-    return maybeArrayA;
-  }
-  return maybeArrayA.concat(maybeArrayB);
-}
-
 const stdTypeMap = keyMap(
   specifiedScalarTypes.concat(introspectionTypes),
   type => type.name,