diff --git a/webauthn-server-core/src/main/java/com/yubico/webauthn/data/AuthenticatorAttachment.java b/webauthn-server-core/src/main/java/com/yubico/webauthn/data/AuthenticatorAttachment.java
index 96c666659..d5d338b42 100644
--- a/webauthn-server-core/src/main/java/com/yubico/webauthn/data/AuthenticatorAttachment.java
+++ b/webauthn-server-core/src/main/java/com/yubico/webauthn/data/AuthenticatorAttachment.java
@@ -26,7 +26,6 @@
 
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonValue;
-import java.util.Optional;
 import java.util.stream.Stream;
 import lombok.AllArgsConstructor;
 import lombok.Getter;
@@ -73,18 +72,8 @@ public enum AuthenticatorAttachment {
 
   @JsonValue @Getter @NonNull private final String value;
 
-  private static Optional<AuthenticatorAttachment> fromString(@NonNull String value) {
-    return Stream.of(values()).filter(v -> v.value.equals(value)).findAny();
-  }
-
   @JsonCreator
   private static AuthenticatorAttachment fromJsonString(@NonNull String value) {
-    return fromString(value)
-        .orElseThrow(
-            () ->
-                new IllegalArgumentException(
-                    String.format(
-                        "Unknown %s value: %s",
-                        AuthenticatorAttachment.class.getSimpleName(), value)));
+    return Stream.of(values()).filter(v -> v.value.equals(value)).findAny().orElse(null);
   }
 }
diff --git a/webauthn-server-core/src/test/scala/com/yubico/webauthn/data/EnumsSpec.scala b/webauthn-server-core/src/test/scala/com/yubico/webauthn/data/EnumsSpec.scala
index 8e35e8558..dbe5ca609 100644
--- a/webauthn-server-core/src/test/scala/com/yubico/webauthn/data/EnumsSpec.scala
+++ b/webauthn-server-core/src/test/scala/com/yubico/webauthn/data/EnumsSpec.scala
@@ -61,11 +61,10 @@ class EnumsSpec
 
   describe("AuthenticatorAttachment") {
     describe("can be parsed from JSON") {
-      it("but throws IllegalArgumentException for unknown values.") {
-        val result = Try(
+      it("and ignores for unknown values.") {
+        val result =
           json.readValue("\"foo\"", classOf[AuthenticatorAttachment])
-        )
-        result.failed.get.getCause shouldBe an[IllegalArgumentException]
+        result should be(null)
       }
     }
   }
diff --git a/webauthn-server-core/src/test/scala/com/yubico/webauthn/data/JsonIoSpec.scala b/webauthn-server-core/src/test/scala/com/yubico/webauthn/data/JsonIoSpec.scala
index 29cfa491f..0fd026f9b 100644
--- a/webauthn-server-core/src/test/scala/com/yubico/webauthn/data/JsonIoSpec.scala
+++ b/webauthn-server-core/src/test/scala/com/yubico/webauthn/data/JsonIoSpec.scala
@@ -41,12 +41,13 @@ import com.yubico.webauthn.extension.appid.Generators._
 import org.junit.runner.RunWith
 import org.scalacheck.Arbitrary
 import org.scalacheck.Arbitrary.arbitrary
-import org.scalacheck.Gen
 import org.scalatest.funspec.AnyFunSpec
 import org.scalatest.matchers.should.Matchers
 import org.scalatestplus.junit.JUnitRunner
 import org.scalatestplus.scalacheck.ScalaCheckDrivenPropertyChecks
 
+import scala.jdk.OptionConverters.RichOptional
+
 @RunWith(classOf[JUnitRunner])
 class JsonIoSpec
     extends AnyFunSpec
@@ -351,15 +352,16 @@ class JsonIoSpec
       )
     }
 
-    it("allows and ignores an authenticatorAttachment attribute.") {
+    it(
+      "allows an authenticatorAttachment attribute, but ignores unknown values."
+    ) {
       def test[P <: PublicKeyCredential[_, _]](tpe: TypeReference[P])(implicit
           a: Arbitrary[P]
       ): Unit = {
         forAll(
           a.arbitrary,
-          Gen.oneOf(
-            arbitrary[AuthenticatorAttachment].map(_.getValue),
-            arbitrary[String],
+          arbitrary[String].suchThat(s =>
+            !AuthenticatorAttachment.values.map(_.getValue).contains(s)
           ),
         ) { (value: P, authenticatorAttachment: String) =>
           val tree: ObjectNode = json.valueToTree(value)
@@ -370,8 +372,37 @@ class JsonIoSpec
           val encoded = json.writeValueAsString(tree)
           println(authenticatorAttachment)
           val decoded = json.readValue(encoded, tpe)
+          decoded.getAuthenticatorAttachment.asScala should be(None)
+        }
+
+        forAll(
+          a.arbitrary,
+          arbitrary[AuthenticatorAttachment],
+        ) { (value: P, authenticatorAttachment: AuthenticatorAttachment) =>
+          val tree: ObjectNode = json.valueToTree(value)
+          tree.set(
+            "authenticatorAttachment",
+            new TextNode(authenticatorAttachment.getValue),
+          )
+          val encoded = json.writeValueAsString(tree)
+          println(authenticatorAttachment)
+          val decoded = json.readValue(encoded, tpe)
+
+          decoded.getAuthenticatorAttachment.asScala should equal(
+            Some(authenticatorAttachment)
+          )
+        }
+
+        forAll(
+          a.arbitrary
+        ) { (value: P) =>
+          val tree: ObjectNode = json.valueToTree(
+            value.toBuilder.authenticatorAttachment(null).build()
+          )
+          val encoded = json.writeValueAsString(tree)
+          val decoded = json.readValue(encoded, tpe)
 
-          decoded should equal(value)
+          decoded.getAuthenticatorAttachment.asScala should be(None)
         }
       }