diff --git a/lib/DefaultResolver.php b/lib/DefaultResolver.php
index a36a857..017ec0c 100644
--- a/lib/DefaultResolver.php
+++ b/lib/DefaultResolver.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types = 1);
 
 namespace Amp\Dns;
 
diff --git a/lib/NoRecordException.php b/lib/NoRecordException.php
index e20426e..93dcd6d 100644
--- a/lib/NoRecordException.php
+++ b/lib/NoRecordException.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types = 1);
 
 namespace Amp\Dns;
 
diff --git a/lib/Record.php b/lib/Record.php
index 9f60a18..ea0dd1d 100644
--- a/lib/Record.php
+++ b/lib/Record.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types = 1);
 
 namespace Amp\Dns;
 
diff --git a/lib/ResolutionException.php b/lib/ResolutionException.php
index e11b880..e7f2f80 100644
--- a/lib/ResolutionException.php
+++ b/lib/ResolutionException.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types = 1);
 
 namespace Amp\Dns;
 
diff --git a/lib/Resolver.php b/lib/Resolver.php
index cba109a..e8c737b 100644
--- a/lib/Resolver.php
+++ b/lib/Resolver.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types = 1);
 
 namespace Amp\Dns;
 
diff --git a/lib/TimeoutException.php b/lib/TimeoutException.php
index a013a8a..4da5349 100644
--- a/lib/TimeoutException.php
+++ b/lib/TimeoutException.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types = 1);
 
 namespace Amp\Dns;
 
diff --git a/lib/constants.php b/lib/constants.php
index cc94839..a1b85e2 100644
--- a/lib/constants.php
+++ b/lib/constants.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types = 1);
 
 namespace Amp\Dns;
 
diff --git a/lib/functions.php b/lib/functions.php
index cfa33d8..56f759a 100644
--- a/lib/functions.php
+++ b/lib/functions.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types = 1);
 
 namespace Amp\Dns;
 
diff --git a/test/IntegrationTest.php b/test/IntegrationTest.php
index 2180bfb..3236706 100644
--- a/test/IntegrationTest.php
+++ b/test/IntegrationTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types = 1);
 
 namespace Amp\Dns\Test;
 
diff --git a/test/ResolvConfTest.php b/test/ResolvConfTest.php
index 73bd20d..6dc4aa6 100644
--- a/test/ResolvConfTest.php
+++ b/test/ResolvConfTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types = 1);
 
 namespace Amp\Dns\Test;