diff --git a/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/jackson2/StdConverters.java b/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/jackson2/StdConverters.java
index ea452cb3169..6aec556b465 100644
--- a/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/jackson2/StdConverters.java
+++ b/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/jackson2/StdConverters.java
@@ -83,7 +83,7 @@ public AuthorizationGrantType convert(JsonNode jsonNode) {
 			if (AuthorizationGrantType.PASSWORD.getValue().equalsIgnoreCase(value)) {
 				return AuthorizationGrantType.PASSWORD;
 			}
-			return null;
+			return new AuthorizationGrantType(value);
 		}
 
 	}
diff --git a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/jackson2/OAuth2AuthorizedClientMixinTests.java b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/jackson2/OAuth2AuthorizedClientMixinTests.java
index 2728942de70..54532ba4adb 100644
--- a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/jackson2/OAuth2AuthorizedClientMixinTests.java
+++ b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/jackson2/OAuth2AuthorizedClientMixinTests.java
@@ -32,6 +32,7 @@
 import org.springframework.security.oauth2.client.OAuth2AuthorizedClient;
 import org.springframework.security.oauth2.client.registration.ClientRegistration;
 import org.springframework.security.oauth2.client.registration.TestClientRegistrations;
+import org.springframework.security.oauth2.core.AuthorizationGrantType;
 import org.springframework.security.oauth2.core.OAuth2AccessToken;
 import org.springframework.security.oauth2.core.OAuth2RefreshToken;
 import org.springframework.security.oauth2.core.TestOAuth2AccessTokens;
@@ -69,6 +70,7 @@ public void setup() {
 		providerConfigurationMetadata.put("config2", "value2");
 		// @formatter:off
 		this.clientRegistrationBuilder = TestClientRegistrations.clientRegistration()
+                .authorizationGrantType(new AuthorizationGrantType("self-defined"))
 				.scope("read", "write")
 				.providerConfigurationMetadata(providerConfigurationMetadata);
 		// @formatter:on