diff --git a/packages/babel-core/src/config/option-assertions.js b/packages/babel-core/src/config/option-assertions.js
index 12aa4460c6..01c65e4e14 100644
--- a/packages/babel-core/src/config/option-assertions.js
+++ b/packages/babel-core/src/config/option-assertions.js
@@ -12,30 +12,36 @@ import type {
   RootInputSourceMapOption,
 } from "./options";
 
-export function assertSourceMaps(key: string, value: mixed): ?SourceMapsOption {
+export function assertSourceMaps(
+  key: string,
+  value: mixed,
+): SourceMapsOption | void {
   if (
-    value != null &&
+    value !== undefined &&
     typeof value !== "boolean" &&
     value !== "inline" &&
     value !== "both"
   ) {
     throw new Error(
-      `.${key} must be a boolean, "inline", "both", null, or undefined`,
+      `.${key} must be a boolean, "inline", "both", or undefined`,
     );
   }
   return value;
 }
 
-export function assertCompact(key: string, value: mixed): ?CompactOption {
-  if (value != null && typeof value !== "boolean" && value !== "auto") {
-    throw new Error(`.${key} must be a boolean, "auto", null, or undefined`);
+export function assertCompact(key: string, value: mixed): CompactOption | void {
+  if (value !== undefined && typeof value !== "boolean" && value !== "auto") {
+    throw new Error(`.${key} must be a boolean, "auto", or undefined`);
   }
   return value;
 }
 
-export function assertSourceType(key: string, value: mixed): ?SourceTypeOption {
-  if (value != null && value !== "module" && value !== "script") {
-    throw new Error(`.${key} must be "module", "script", null, or undefined`);
+export function assertSourceType(
+  key: string,
+  value: mixed,
+): SourceTypeOption | void {
+  if (value !== undefined && value !== "module" && value !== "script") {
+    throw new Error(`.${key} must be "module", "script", or undefined`);
   }
   return value;
 }
@@ -43,48 +49,49 @@ export function assertSourceType(key: string, value: mixed): ?SourceTypeOption {
 export function assertInputSourceMap(
   key: string,
   value: mixed,
-): ?RootInputSourceMapOption {
+): RootInputSourceMapOption | void {
   if (
-    value != null &&
+    value !== undefined &&
     typeof value !== "boolean" &&
-    typeof value !== "object"
+    (typeof value !== "object" || !value)
   ) {
-    throw new Error(
-      ".inputSourceMap must be a boolean, object, null, or undefined",
-    );
+    throw new Error(".inputSourceMap must be a boolean, object, or undefined");
   }
   return value;
 }
 
-export function assertString(key: string, value: mixed): ?string {
-  if (value != null && typeof value !== "string") {
-    throw new Error(`.${key} must be a string, null, or undefined`);
+export function assertString(key: string, value: mixed): string | void {
+  if (value !== undefined && typeof value !== "string") {
+    throw new Error(`.${key} must be a string, or undefined`);
   }
   return value;
 }
 
-export function assertFunction(key: string, value: mixed): ?Function {
-  if (value != null && typeof value !== "function") {
-    throw new Error(`.${key} must be a function, null, or undefined`);
+export function assertFunction(key: string, value: mixed): Function | void {
+  if (value !== undefined && typeof value !== "function") {
+    throw new Error(`.${key} must be a function, or undefined`);
   }
   return value;
 }
 
-export function assertBoolean(key: string, value: mixed): ?boolean {
-  if (value != null && typeof value !== "boolean") {
-    throw new Error(`.${key} must be a boolean, null, or undefined`);
+export function assertBoolean(key: string, value: mixed): boolean | void {
+  if (value !== undefined && typeof value !== "boolean") {
+    throw new Error(`.${key} must be a boolean, or undefined`);
   }
   return value;
 }
 
-export function assertObject(key: string, value: mixed): ?{} {
-  if (value != null && (typeof value !== "object" || Array.isArray(value))) {
-    throw new Error(`.${key} must be an object, null, or undefined`);
+export function assertObject(key: string, value: mixed): {} | void {
+  if (
+    value !== undefined &&
+    (typeof value !== "object" || Array.isArray(value) || !value)
+  ) {
+    throw new Error(`.${key} must be an object, or undefined`);
   }
   return value;
 }
 
-export function assertIgnoreList(key: string, value: mixed): ?IgnoreList {
+export function assertIgnoreList(key: string, value: mixed): IgnoreList | void {
   const arr = assertArray(key, value);
   if (arr) {
     arr.forEach((item, i) => assertIgnoreItem(key, i, item));
@@ -102,13 +109,13 @@ function assertIgnoreItem(
     !(value instanceof RegExp)
   ) {
     throw new Error(
-      `.${key}[${index}] must be an array of string/Funtion/RegExp values, or null, or undefined`,
+      `.${key}[${index}] must be an array of string/Funtion/RegExp values, or or undefined`,
     );
   }
   return value;
 }
 
-export function assertPluginList(key: string, value: mixed): ?PluginList {
+export function assertPluginList(key: string, value: mixed): PluginList | void {
   const arr = assertArray(key, value);
   if (arr) {
     // Loop instead of using `.map` in order to preserve object identity
@@ -135,9 +142,7 @@ function assertPluginItem(
     if (value.length === 2) {
       const opts = value[1];
       if (opts != null && (typeof opts !== "object" || Array.isArray(opts))) {
-        throw new Error(
-          `.${key}[${index}][1] must be an object, null, or undefined`,
-        );
+        throw new Error(`.${key}[${index}][1] must be an object, or undefined`);
       }
     }
   } else {
@@ -168,7 +173,7 @@ function assertPluginTarget(
 
 function assertArray(key: string, value: mixed): ?$ReadOnlyArray<mixed> {
   if (value != null && !Array.isArray(value)) {
-    throw new Error(`.${key} must be an array, null, or undefined`);
+    throw new Error(`.${key} must be an array, or undefined`);
   }
   return value;
 }
diff --git a/packages/babel-core/src/config/options.js b/packages/babel-core/src/config/options.js
index 75a91388f9..c46ca0d3df 100644
--- a/packages/babel-core/src/config/options.js
+++ b/packages/babel-core/src/config/options.js
@@ -126,54 +126,54 @@ const COMMON_VALIDATORS: ValidatorSet = {
   >),
 };
 export type ValidatedOptions = {
-  filename?: ?string,
-  filenameRelative?: ?string,
-  babelrc?: ?boolean,
-  code?: ?boolean,
-  ast?: ?boolean,
-  inputSourceMap?: ?RootInputSourceMapOption,
+  filename?: string,
+  filenameRelative?: string,
+  babelrc?: boolean,
+  code?: boolean,
+  ast?: boolean,
+  inputSourceMap?: RootInputSourceMapOption,
 
-  extends?: ?string,
-  env?: ?EnvSet<ValidatedOptions>,
-  ignore?: ?IgnoreList,
-  only?: ?IgnoreList,
+  extends?: string,
+  env?: EnvSet<ValidatedOptions>,
+  ignore?: IgnoreList,
+  only?: IgnoreList,
 
-  presets?: ?PluginList,
-  plugins?: ?PluginList,
-  passPerPreset?: ?boolean,
+  presets?: PluginList,
+  plugins?: PluginList,
+  passPerPreset?: boolean,
 
   // Options for @babel/generator
-  retainLines?: ?boolean,
-  comments?: ?boolean,
-  shouldPrintComment?: ?Function,
-  compact?: ?CompactOption,
-  minified?: ?boolean,
-  auxiliaryCommentBefore?: ?string,
-  auxiliaryCommentAfter?: ?string,
+  retainLines?: boolean,
+  comments?: boolean,
+  shouldPrintComment?: Function,
+  compact?: CompactOption,
+  minified?: boolean,
+  auxiliaryCommentBefore?: string,
+  auxiliaryCommentAfter?: string,
 
   // Parser
-  sourceType?: ?SourceTypeOption,
+  sourceType?: SourceTypeOption,
 
-  wrapPluginVisitorMethod?: ?Function,
-  highlightCode?: ?boolean,
+  wrapPluginVisitorMethod?: Function,
+  highlightCode?: boolean,
 
   // Sourcemap generation options.
-  sourceMaps?: ?SourceMapsOption,
-  sourceMap?: ?SourceMapsOption,
-  sourceMapTarget?: ?string,
-  sourceFileName?: ?string,
-  sourceRoot?: ?string,
+  sourceMaps?: SourceMapsOption,
+  sourceMap?: SourceMapsOption,
+  sourceMapTarget?: string,
+  sourceFileName?: string,
+  sourceRoot?: string,
 
   // AMD/UMD/SystemJS module naming options.
-  getModuleId?: ?Function,
-  moduleRoot?: ?string,
-  moduleIds?: ?boolean,
-  moduleId?: ?string,
+  getModuleId?: Function,
+  moduleRoot?: string,
+  moduleIds?: boolean,
+  moduleId?: string,
 
   // Deprecate top level parserOpts
-  parserOpts?: ?{},
+  parserOpts?: {},
   // Deprecate top level generatorOpts
-  generatorOpts?: ?{},
+  generatorOpts?: {},
 };
 
 export type EnvSet<T> = {
@@ -241,7 +241,7 @@ function assertNoDuplicateSourcemap(opts: {}): void {
   }
 }
 
-function assertEnvSet(key: string, value: mixed): ?EnvSet<ValidatedOptions> {
+function assertEnvSet(key: string, value: mixed): EnvSet<ValidatedOptions> {
   const obj = assertObject(key, value);
   if (obj) {
     // Validate but don't copy the .env object in order to preserve