diff --git a/java/com/google/gerrit/plugins/codeowners/backend/CodeOwnerResolver.java b/java/com/google/gerrit/plugins/codeowners/backend/CodeOwnerResolver.java
index cf9a16b..3cb377b 100644
--- a/java/com/google/gerrit/plugins/codeowners/backend/CodeOwnerResolver.java
+++ b/java/com/google/gerrit/plugins/codeowners/backend/CodeOwnerResolver.java
@@ -370,6 +370,8 @@
    *     {@code false}
    */
   public boolean isEmailDomainAllowed(String email) {
+    requireNonNull(email, "email");
+
     ImmutableSet<String> allowedEmailDomains =
         codeOwnersPluginConfiguration.getAllowedEmailDomains();
     if (allowedEmailDomains.isEmpty()) {
diff --git a/javatests/com/google/gerrit/plugins/codeowners/backend/CodeOwnerResolverTest.java b/javatests/com/google/gerrit/plugins/codeowners/backend/CodeOwnerResolverTest.java
index 0ac0deb..bd1bad1 100644
--- a/javatests/com/google/gerrit/plugins/codeowners/backend/CodeOwnerResolverTest.java
+++ b/javatests/com/google/gerrit/plugins/codeowners/backend/CodeOwnerResolverTest.java
@@ -322,6 +322,14 @@
   }
 
   @Test
+  public void isEmailDomainAllowedRequiresEmailToBeNonNull() throws Exception {
+    NullPointerException npe =
+        assertThrows(
+            NullPointerException.class, () -> codeOwnerResolver.get().isEmailDomainAllowed(null));
+    assertThat(npe).hasMessageThat().isEqualTo("email");
+  }
+
+  @Test
   @GerritConfig(
       name = "plugin.code-owners.allowedEmailDomain",
       values = {"example.com", "example.net"})