diff --git a/.travis.yml b/.travis.yml
index 3843bdd..d36423f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,14 +10,6 @@ sudo: false
 matrix:
   fast_finish: true
   include:
-    - php: 5.4
-      env: polyfill='true'
-    - php: 5.4
-      env: polyfill='false'
-    - php: 5.5
-      env: polyfill='true'
-    - php: 5.5
-      env: polyfill='false'
     - php: 5.6
       env: polyfill='true'
     - php: 5.6
@@ -30,8 +22,11 @@ matrix:
       env: polyfill='true'
     - php: 7.1
       env: polyfill='false'
-
-install: travis_retry composer install --no-interaction --prefer-source
+      env: polyfill='false'
+    - php: 7.2
+      env: polyfill='true'
+    - php: 7.2
+      env: polyfill='false'
 
 before_script:
   - travis_retry composer self-update
diff --git a/composer.json b/composer.json
index 092989f..989b526 100644
--- a/composer.json
+++ b/composer.json
@@ -15,11 +15,11 @@
         }
     ],
     "require": {
-        "php": ">=5.4.0",
+        "php": ">=5.6.0",
         "symfony/polyfill-mbstring": "~1.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "~4.0"
+        "phpunit/phpunit": "^5.7 || ^6.5"
     },
     "support": {
         "issues": "https://github.com/danielstjules/Stringy/issues",
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index fd7d0e7..353a8cd 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -6,9 +6,15 @@
          syntaxCheck="false">
     <testsuites>
         <testsuite name="Stringy">
-            <file>tests/StringyTest.php</file>
-            <file>tests/StaticStringyTest.php</file>
-            <file phpVersion="5.6.0">tests/CreateTest.php</file>
+            <directory>tests</directory>
         </testsuite>
     </testsuites>
+    <filter>
+        <whitelist>
+            <directory suffix=".php">src</directory>
+            <exclude>
+                <directory suffix="include.php">src</directory>
+            </exclude>
+        </whitelist>
+    </filter>
 </phpunit>
diff --git a/tests/CreateTest.php b/tests/CreateTest.php
index 5e211cd..2649e7d 100644
--- a/tests/CreateTest.php
+++ b/tests/CreateTest.php
@@ -1,8 +1,9 @@
 <?php
 
 use function Stringy\create as s;
+use PHPUnit\Framework\TestCase;
 
-class CreateTestCase extends PHPUnit_Framework_TestCase
+class CreateTestCase extends TestCase
 {
     public function testCreate()
     {
diff --git a/tests/StaticStringyTest.php b/tests/StaticStringyTest.php
index 67c3946..159e5bd 100644
--- a/tests/StaticStringyTest.php
+++ b/tests/StaticStringyTest.php
@@ -1,8 +1,9 @@
 <?php
 
 use Stringy\StaticStringy as S;
+use PHPUnit\Framework\TestCase;
 
-class StaticStringyTestCase extends PHPUnit_Framework_TestCase
+class StaticStringyTestCase extends TestCase
 {
     /**
      * @expectedException BadMethodCallException
@@ -25,6 +26,13 @@ public function testInvocation()
         $this->assertInternalType('string', $result);
     }
 
+    public function testEmptyStringToBoolean()
+    {
+        $result = S::toBoolean('');
+        $this->assertFalse($result);
+        $this->assertInternalType('bool', $result);
+    }
+
     public function testPartialArgsInvocation()
     {
         $result = S::slice('foobar', 0, 3);
diff --git a/tests/StringyTest.php b/tests/StringyTest.php
index 97d5502..4c17f33 100644
--- a/tests/StringyTest.php
+++ b/tests/StringyTest.php
@@ -1,8 +1,9 @@
 <?php
 
 use Stringy\Stringy as S;
+use PHPUnit\Framework\TestCase;
 
-class StringyTestCase extends PHPUnit_Framework_TestCase
+class StringyTestCase extends TestCase
 {
     /**
      * Asserts that a variable is of a Stringy instance.
@@ -849,6 +850,12 @@ public function startsWithProvider()
         ];
     }
 
+    public function testStartsWithAnyOnNullSubstring()
+    {
+        $stringy = S::create('foo bar', 'UTF-8');
+        $this->assertFalse($stringy->startsWithAny(null));
+    }
+
     /**
      * @dataProvider startsWithProviderAny()
      */