diff --git a/buildSrc/build.gradle b/buildSrc/build.gradle
new file mode 100644
index 0000000000000000000000000000000000000000..437b1aa6308681f5c32e29f445e012321a88c052
--- /dev/null
+++ b/buildSrc/build.gradle
@@ -0,0 +1,7 @@
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    api 'org.briarproject:null-safety:0.1'
+}
\ No newline at end of file
diff --git a/buildSrc/src/main/java/org/briarproject/onionwrapper/OS.java b/buildSrc/src/main/java/org/briarproject/onionwrapper/OS.java
index cc48f22bff21d16296fe62f263b62cc2cc5747b2..7f9e20554cc3e4304225388c75519f07e1769db6 100644
--- a/buildSrc/src/main/java/org/briarproject/onionwrapper/OS.java
+++ b/buildSrc/src/main/java/org/briarproject/onionwrapper/OS.java
@@ -1,5 +1,8 @@
 package org.briarproject.onionwrapper;
 
+import org.briarproject.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
 public enum OS {
 	Linux("linux"),
 	Windows("windows"),
diff --git a/buildSrc/src/main/java/org/briarproject/onionwrapper/OsUtils.java b/buildSrc/src/main/java/org/briarproject/onionwrapper/OsUtils.java
index 5b4ee27d11bf74127bb3dbfc6ee62f78f9e28b85..605f5c9f9d4fe9d0856650b9e9a4eda2b3ed6fda 100644
--- a/buildSrc/src/main/java/org/briarproject/onionwrapper/OsUtils.java
+++ b/buildSrc/src/main/java/org/briarproject/onionwrapper/OsUtils.java
@@ -1,7 +1,10 @@
 package org.briarproject.onionwrapper;
 
+import org.briarproject.nullsafety.NotNullByDefault;
+
 import static org.briarproject.onionwrapper.StringUtils.startsWithIgnoreCase;
 
+@NotNullByDefault
 public class OsUtils {
 
 	public static final OS currentOS;
diff --git a/buildSrc/src/main/java/org/briarproject/onionwrapper/StringUtils.java b/buildSrc/src/main/java/org/briarproject/onionwrapper/StringUtils.java
index 2a9b5db89dc5fbb1183c311b5b4656d4f8746814..bd03d2f2e05b651336e136e08b9f15dd49934bfd 100644
--- a/buildSrc/src/main/java/org/briarproject/onionwrapper/StringUtils.java
+++ b/buildSrc/src/main/java/org/briarproject/onionwrapper/StringUtils.java
@@ -1,5 +1,8 @@
 package org.briarproject.onionwrapper;
 
+import org.briarproject.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
 public class StringUtils {
 
 	// see https://stackoverflow.com/a/38947571