diff --git a/src/MDCurrenciesSet.php b/src/MDCurrenciesSet.php
index cf5f2fb..6d93204 100644
--- a/src/MDCurrenciesSet.php
+++ b/src/MDCurrenciesSet.php
@@ -4,6 +4,7 @@
  *
  * @author Joshua Ramon Enslin <joshua@museum-digital.de>
  */
+declare(strict_types = 1);
 
 /**
  * Class containing available currencies.
diff --git a/src/MDEventsSet.php b/src/MDEventsSet.php
index ef34350..1a06f9f 100644
--- a/src/MDEventsSet.php
+++ b/src/MDEventsSet.php
@@ -4,6 +4,7 @@
  *
  * @author Joshua Ramon Enslin <joshua@museum-digital.de>
  */
+declare(strict_types = 1);
 
 /**
  * Class containing available units for events.
diff --git a/src/MDLanguagesSet.php b/src/MDLanguagesSet.php
index 3361443..416d68a 100644
--- a/src/MDLanguagesSet.php
+++ b/src/MDLanguagesSet.php
@@ -4,6 +4,7 @@
  *
  * @author Joshua Ramon Enslin <joshua@museum-digital.de>
  */
+declare(strict_types = 1);
 
 /**
  * Class containing static functions for getting available languages.
diff --git a/src/MDLicensesSet.php b/src/MDLicensesSet.php
index 28623ad..6176658 100644
--- a/src/MDLicensesSet.php
+++ b/src/MDLicensesSet.php
@@ -4,11 +4,12 @@
  *
  * @author Joshua Ramon Enslin <joshua@museum-digital.de>
  */
+declare(strict_types = 1);
 
 /**
  * Class containing static functions for getting available licenses on md.
  */
-class MDLicensesSet {
+class MDLicensesSet extends MDValueSet {
 
     const AVAILABLE_LICENSES = [
 
diff --git a/src/MDMarkingTypesSet.php b/src/MDMarkingTypesSet.php
index ba41ce9..6500b0b 100644
--- a/src/MDMarkingTypesSet.php
+++ b/src/MDMarkingTypesSet.php
@@ -4,6 +4,7 @@
  *
  * @author Joshua Ramon Enslin <joshua@museum-digital.de>
  */
+declare(strict_types = 1);
 
 /**
  * Class containing available marking types.
diff --git a/src/MDObjectPositionsSet.php b/src/MDObjectPositionsSet.php
index 0bfc2f4..ba39fe2 100644
--- a/src/MDObjectPositionsSet.php
+++ b/src/MDObjectPositionsSet.php
@@ -4,6 +4,7 @@
  *
  * @author Joshua Ramon Enslin <joshua@museum-digital.de>
  */
+declare(strict_types = 1);
 
 /**
  * Class containing available positions on an object.
diff --git a/src/MDPlacetypesSet.php b/src/MDPlacetypesSet.php
index a8988f3..a59df53 100644
--- a/src/MDPlacetypesSet.php
+++ b/src/MDPlacetypesSet.php
@@ -4,6 +4,7 @@
  *
  * @author Joshua Ramon Enslin <joshua@museum-digital.de>
  */
+declare(strict_types = 1);
 
 /**
  * Class containing available place types.
diff --git a/src/MDUnitsSet.php b/src/MDUnitsSet.php
index aa0c47e..8635fee 100644
--- a/src/MDUnitsSet.php
+++ b/src/MDUnitsSet.php
@@ -4,6 +4,7 @@
  *
  * @author Joshua Ramon Enslin <joshua@museum-digital.de>
  */
+declare(strict_types = 1);
 
 /**
  * Class containing available units for weights, lengths etc.
diff --git a/src/MDValueSet.php b/src/MDValueSet.php
index f1c50c7..b9f0ac9 100644
--- a/src/MDValueSet.php
+++ b/src/MDValueSet.php
@@ -4,6 +4,7 @@
  *
  * @author Joshua Ramon Enslin <joshua@museum-digital.de>
  */
+declare(strict_types = 1);
 
 /**
  * Generic class for value sets.