diff --git a/src/PhoneNumber.php b/src/PhoneNumber.php
index ea2f7a2..6751598 100644
--- a/src/PhoneNumber.php
+++ b/src/PhoneNumber.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Brick\PhoneNumber;
 
 use libphonenumber\NumberParseException;
diff --git a/src/PhoneNumberException.php b/src/PhoneNumberException.php
index 2367091..2e756ad 100644
--- a/src/PhoneNumberException.php
+++ b/src/PhoneNumberException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Brick\PhoneNumber;
 
 /**
diff --git a/src/PhoneNumberFormat.php b/src/PhoneNumberFormat.php
index be226b0..b1b8da6 100644
--- a/src/PhoneNumberFormat.php
+++ b/src/PhoneNumberFormat.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Brick\PhoneNumber;
 
 /**
diff --git a/src/PhoneNumberParseException.php b/src/PhoneNumberParseException.php
index 5b25b2e..76493eb 100644
--- a/src/PhoneNumberParseException.php
+++ b/src/PhoneNumberParseException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Brick\PhoneNumber;
 
 /**
diff --git a/src/PhoneNumberType.php b/src/PhoneNumberType.php
index 57e315c..44f7615 100644
--- a/src/PhoneNumberType.php
+++ b/src/PhoneNumberType.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Brick\PhoneNumber;
 
 /**