diff --git a/README.md b/README.md
index 6255165..f8041c5 100644
--- a/README.md
+++ b/README.md
@@ -86,8 +86,7 @@ Node's `process.env` only stores strings, but sometimes you want to retrieve oth
 URL, email address). To these ends, the following validation functions are available:
 
 - `str()` - Passes string values through, will ensure a value is present unless a
-  `default` value is given. Note that an empty string is considered a valid value -
-  if this is undesirable you can easily create your own validator (see below)
+  `default` value is given. Note that an empty string is not considered a valid value
 - `bool()` - Parses env var strings `"1", "0", "true", "false", "t", "f", "yes", "no", "on", "off"` into booleans
 - `num()` - Parses an env var (eg. `"42", "0.23", "1e5"`) into a Number
 - `email()` - Ensures an env var is an email address
diff --git a/src/core.ts b/src/core.ts
index a53fa52..fedc0b2 100644
--- a/src/core.ts
+++ b/src/core.ts
@@ -1,6 +1,6 @@
 import { EnvError, EnvMissingError } from './errors'
-import type { CleanOptions, SpecsOutput, Spec, ValidatorSpec } from './types'
 import { defaultReporter } from './reporter'
+import type { CleanOptions, Spec, SpecsOutput, ValidatorSpec } from './types'
 
 /**
  * Validate a single env var, given a spec object
@@ -40,8 +40,17 @@ export function formatSpecDescription<T>(spec: Spec<T>) {
   return `${spec.desc}${egText}${docsText}`
 }
 
-const readRawEnvValue = <T>(env: unknown, k: keyof T | 'NODE_ENV'): string | T[keyof T] => {
-  return (env as any)[k]
+const readRawEnvValue = <T>(
+  env: unknown,
+  k: keyof T | 'NODE_ENV',
+): string | undefined | T[keyof T] => {
+  const result = (env as any)[k]
+
+  if (typeof result == 'string' && !result.trim()) {
+    return undefined
+  }
+
+  return result
 }
 
 /**
diff --git a/tests/requiredWhen.test.ts b/tests/requiredWhen.test.ts
index 2389d6e..e1ac8f4 100644
--- a/tests/requiredWhen.test.ts
+++ b/tests/requiredWhen.test.ts
@@ -3,8 +3,8 @@ import { bool, cleanEnv, defaultReporter, EnvMissingError, num, EnvError } from
 import { formatSpecDescription } from '../src/core'
 
 vi.mock('../src/reporter')
-const mockedDefaultReporter: vi.Mock = <vi.Mock<typeof defaultReporter>>defaultReporter;
-mockedDefaultReporter.mockImplementation(() => { })
+const mockedDefaultReporter: vi.Mock = <vi.Mock<typeof defaultReporter>>defaultReporter
+mockedDefaultReporter.mockImplementation(() => {})
 
 describe('requiredWhen', () => {
   beforeEach(() => {
@@ -13,7 +13,7 @@ describe('requiredWhen', () => {
   test("isn't required", () => {
     cleanEnv(
       {
-        autoExtractId: "true",
+        autoExtractId: 'true',
       },
       {
         autoExtractId: bool(),
@@ -29,14 +29,54 @@ describe('requiredWhen', () => {
         autoExtractId: true,
         id: undefined,
       },
-      errors: {}
+      errors: {},
+    })
+  })
+
+  test("isn't required but empty string provided", () => {
+    cleanEnv(
+      {
+        id: '',
+      },
+      {
+        id: num({
+          default: undefined,
+          requiredWhen: () => false,
+        }),
+      },
+    )
+    expect(mockedDefaultReporter).toHaveBeenCalledTimes(1)
+    expect(mockedDefaultReporter).toHaveBeenCalledWith({
+      env: {
+        id: undefined,
+      },
+      errors: {},
+    })
+
+    cleanEnv(
+      {
+        autoExtractId: '',
+      },
+      {
+        autoExtractId: bool({
+          default: undefined,
+          requiredWhen: () => false,
+        }),
+      },
+    )
+    expect(mockedDefaultReporter).toHaveBeenCalledTimes(2)
+    expect(mockedDefaultReporter).toHaveBeenCalledWith({
+      env: {
+        autoExtractId: undefined,
+      },
+      errors: {},
     })
   })
 
   test('required but not provided', () => {
     cleanEnv(
       {
-        autoExtractId: "false",
+        autoExtractId: 'false',
       },
       {
         autoExtractId: bool(),
@@ -68,8 +108,8 @@ describe('requiredWhen', () => {
   test('required and provided', () => {
     cleanEnv(
       {
-        autoExtractId: "false",
-        id: "123"
+        autoExtractId: 'false',
+        id: '123',
       },
       {
         autoExtractId: bool(),
@@ -92,8 +132,8 @@ describe('requiredWhen', () => {
   test('required but failed to parse', () => {
     cleanEnv(
       {
-        autoExtractId: "false",
-        id: "abc"
+        autoExtractId: 'false',
+        id: 'abc',
       },
       {
         autoExtractId: bool(),
@@ -110,7 +150,7 @@ describe('requiredWhen', () => {
         id: undefined,
       },
       errors: {
-        id: new EnvError(`Invalid number input: "abc"`)
+        id: new EnvError(`Invalid number input: "abc"`),
       },
     })
   })
diff --git a/tests/validators.test.ts b/tests/validators.test.ts
index 60ead85..1ea1d14 100644
--- a/tests/validators.test.ts
+++ b/tests/validators.test.ts
@@ -39,8 +39,19 @@ test('bool() works with various formats', () => {
   const off = cleanEnv({ FOO: 'off' }, { FOO: bool() })
   expect(off).toEqual({ FOO: false })
 
+  expect(function withEmpty() {
+    return cleanEnv({ FOO: '' }, { FOO: bool() }, makeSilent)
+  }).toThrow()
+
   const defaultF = cleanEnv({}, { FOO: bool({ default: false }) })
   expect(defaultF).toEqual({ FOO: false })
+
+  const defaultTWithWhitespace = cleanEnv(
+    { FOO: ' ' },
+    { FOO: bool({ default: true }) },
+    makeSilent,
+  )
+  expect(defaultTWithWhitespace).toEqual({ FOO: true })
 })
 
 test('num()', () => {
@@ -132,8 +143,18 @@ test('url()', () => {
 })
 
 test('str()', () => {
-  const withEmpty = cleanEnv({ FOO: '' }, { FOO: str() })
-  expect(withEmpty).toEqual({ FOO: '' })
+  expect(cleanEnv({ FOO: 'asdf' }, { FOO: str() })).toEqual({ FOO: 'asdf' })
+
+  expect(function withWhitespace() {
+    return cleanEnv({ FOO: ' ' }, { FOO: str() }, makeSilent)
+  }).toThrow()
+
+  const defaultWithWhitespace = cleanEnv(
+    { FOO: ' ' },
+    { FOO: str({ default: 'asdf' }) },
+    makeSilent,
+  )
+  expect(defaultWithWhitespace).toEqual({ FOO: 'asdf' })
 
   expect(() => cleanEnv({ FOO: 42 }, { FOO: str() }, makeSilent)).toThrow()
 })