diff --git a/packages/babel-types/scripts/generators/flow.js b/packages/babel-types/scripts/generators/flow.js
index d82ced365c66..f3d8a85233fd 100644
--- a/packages/babel-types/scripts/generators/flow.js
+++ b/packages/babel-types/scripts/generators/flow.js
@@ -146,7 +146,7 @@ lines.push(
   `declare function isLet(node: BabelNode): boolean`,
   `declare function isNode(node: ?Object): boolean`,
   `declare function isNodesEquivalent(a: any, b: any): boolean`,
-  `declare function isPlaceholderType(placeholderType: ?string, targetType: string): boolean`,
+  `declare function isPlaceholderType(placeholderType: string, targetType: string): boolean`,
   `declare function isReferenced(node: BabelNode, parent: BabelNode, grandparent?: BabelNode): boolean`,
   `declare function isScope(node: BabelNode, parent: BabelNode): boolean`,
   `declare function isSpecifierDefault(specifier: BabelNodeModuleSpecifier): boolean`,
diff --git a/packages/babel-types/scripts/generators/typescript.js b/packages/babel-types/scripts/generators/typescript.js
index 35696923e1ae..6666a277d37e 100644
--- a/packages/babel-types/scripts/generators/typescript.js
+++ b/packages/babel-types/scripts/generators/typescript.js
@@ -153,13 +153,13 @@ lines.push(
   `export function isBlockScoped(node: Node): boolean`,
   `export function isImmutable(node: Node): boolean`,
   `export function isLet(node: Node): boolean`,
-  `export function isNode(node: ?object): boolean`,
+  `export function isNode(node: object | null | undefined): boolean`,
   `export function isNodesEquivalent(a: any, b: any): boolean`,
-  `export function isPlaceholderType(placeholderType: ?string, targetType: string): boolean`,
+  `export function isPlaceholderType(placeholderType: string, targetType: string): boolean`,
   `export function isReferenced(node: Node, parent: Node, grandparent?: Node): boolean`,
   `export function isScope(node: Node, parent: Node): boolean`,
   `export function isSpecifierDefault(specifier: ModuleSpecifier): boolean`,
-  `export function isType(nodetype: ?string, targetType: string): boolean`,
+  `export function isType(nodetype: string | null | undefined, targetType: string): boolean`,
   `export function isValidES3Identifier(name: string): boolean`,
   `export function isValidES3Identifier(name: string): boolean`,
   `export function isValidIdentifier(name: string): boolean`,
diff --git a/packages/babel-types/src/validators/isPlaceholderType.js b/packages/babel-types/src/validators/isPlaceholderType.js
index fa775b7ed803..fac93b274532 100644
--- a/packages/babel-types/src/validators/isPlaceholderType.js
+++ b/packages/babel-types/src/validators/isPlaceholderType.js
@@ -5,7 +5,7 @@ import { PLACEHOLDERS_ALIAS } from "../definitions";
  * Test if a `placeholderType` is a `targetType` or if `targetType` is an alias of `placeholderType`.
  */
 export default function isPlaceholderType(
-  placeholderType: ?string,
+  placeholderType: string,
   targetType: string,
 ): boolean {
   if (placeholderType === targetType) return true;