diff --git a/jgvariant-ostree/pom.xml b/jgvariant-ostree/pom.xml
index 1007ee1..538630b 100644
--- a/jgvariant-ostree/pom.xml
+++ b/jgvariant-ostree/pom.xml
@@ -52,6 +52,11 @@
       <optional>true</optional>
     </dependency>
     <dependency>
+      <groupId>org.jspecify</groupId>
+      <artifactId>jspecify</artifactId>
+      <optional>true</optional>
+    </dependency>
+    <dependency>
       <groupId>org.apiguardian</groupId>
       <artifactId>apiguardian-api</artifactId>
       <optional>true</optional>
diff --git a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/ByteString.java b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/ByteString.java
index 3bd8b25..f9309b9 100644
--- a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/ByteString.java
+++ b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/ByteString.java
@@ -10,7 +10,7 @@
 import java.util.HexFormat;
 import java.util.stream.IntStream;
 import java.util.stream.Stream;
-import org.jetbrains.annotations.Nullable;
+import org.jspecify.annotations.Nullable;
 
 /**
  * A wrapper for a {@code byte[]} that implements {@link #equals(Object)}, {@link #hashCode()}, and
diff --git a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/package-info.java b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/package-info.java
index 858a748..63988ce 100644
--- a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/package-info.java
+++ b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/package-info.java
@@ -116,7 +116,9 @@
  * </dl>
  */
 @API(status = Status.EXPERIMENTAL)
+@NullMarked
 package eu.mulk.jgvariant.ostree;
 
 import org.apiguardian.api.API;
 import org.apiguardian.api.API.Status;
+import org.jspecify.annotations.NullMarked;
diff --git a/jgvariant-ostree/src/main/java/module-info.java b/jgvariant-ostree/src/main/java/module-info.java
index cd5f78c..201901c 100644
--- a/jgvariant-ostree/src/main/java/module-info.java
+++ b/jgvariant-ostree/src/main/java/module-info.java
@@ -2,6 +2,8 @@
 //
 // SPDX-License-Identifier: LGPL-3.0-or-later
 
+import org.jspecify.annotations.NullMarked;
+
 /**
  * {@link eu.mulk.jgvariant.core.Decoder} instances for OSTree repositories.
  *
@@ -78,6 +80,7 @@
  * }
  * }
  */
+@NullMarked
 module eu.mulk.jgvariant.ostree {
   requires transitive eu.mulk.jgvariant.core;
 
@@ -86,6 +89,7 @@
   requires static com.google.errorprone.annotations;
   requires static org.apiguardian.api;
   requires static org.jetbrains.annotations;
+  requires static org.jspecify;
 
   exports eu.mulk.jgvariant.ostree;
 }