diff --git a/src/Validator.php b/src/Validator.php
index efea1220..ebfb4f36 100644
--- a/src/Validator.php
+++ b/src/Validator.php
@@ -101,7 +101,10 @@ public function isBoolean()
     {
         return $this->assertNullable(
             static function (string $value) {
-                if ($value === '') {
+                // filter_var() trims its input before matching, so a value that
+                // is empty only after trimming (eg. whitespace-only) would
+                // otherwise be treated as the valid boolean false.
+                if (\trim($value, " \n\r\t\0\x0B") === '') {
                     return false;
                 }
 
diff --git a/tests/Dotenv/ValidatorTest.php b/tests/Dotenv/ValidatorTest.php
index 7d4ed3b5..c9654678 100644
--- a/tests/Dotenv/ValidatorTest.php
+++ b/tests/Dotenv/ValidatorTest.php
@@ -269,6 +269,7 @@ public static function invalidBooleanValuesDataProvider()
             ['INVALID_SOMETHING'],
             ['INVALID_EMPTY'],
             ['INVALID_EMPTY_STRING'],
+            ['INVALID_WHITESPACE'],
             ['INVALID_NULL'],
             ['INVALID_NUMBER_POSITIVE'],
             ['INVALID_NUMBER_NEGATIVE'],
diff --git a/tests/fixtures/env/booleans.env b/tests/fixtures/env/booleans.env
index 522ac456..9fbf00c7 100644
--- a/tests/fixtures/env/booleans.env
+++ b/tests/fixtures/env/booleans.env
@@ -25,6 +25,7 @@ VALID_YESNO_MIXEDCASE_FALSE=No
 INVALID_SOMETHING=something
 INVALID_EMPTY=
 INVALID_EMPTY_STRING=""
+INVALID_WHITESPACE="   "
 INVALID_NULL=null
 INVALID_NUMBER_POSITIVE=2
 INVALID_NUMBER_NEGATIVE=-2