diff --git a/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/endpoint/AbstractWebClientReactiveOAuth2AccessTokenResponseClient.java b/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/endpoint/AbstractWebClientReactiveOAuth2AccessTokenResponseClient.java
index fff6222bb6..0b2403e897 100644
--- a/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/endpoint/AbstractWebClientReactiveOAuth2AccessTokenResponseClient.java
+++ b/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/endpoint/AbstractWebClientReactiveOAuth2AccessTokenResponseClient.java
@@ -115,7 +115,8 @@ public abstract class AbstractWebClientReactiveOAuth2AccessTokenResponseClient<T
 		if (!supportedClientAuthenticationMethod) {
 			throw new IllegalArgumentException(String.format(
 					"This class supports `client_secret_basic`, `client_secret_post`, and `none` by default. Client [%s] is using [%s] instead. Please use a supported client authentication method, or use `set/addParametersConverter` or `set/addHeadersConverter` to supply an instance that supports [%s].",
-					clientRegistration.getRegistrationId(), clientAuthenticationMethod, clientAuthenticationMethod));
+					clientRegistration.getRegistrationId(), clientAuthenticationMethod.getValue(),
+					clientAuthenticationMethod.getValue()));
 		}
 	}
 
diff --git a/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/endpoint/ClientAuthenticationMethodValidatingRequestEntityConverter.java b/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/endpoint/ClientAuthenticationMethodValidatingRequestEntityConverter.java
index 1c6068b08b..0c2537d896 100644
--- a/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/endpoint/ClientAuthenticationMethodValidatingRequestEntityConverter.java
+++ b/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/endpoint/ClientAuthenticationMethodValidatingRequestEntityConverter.java
@@ -41,7 +41,7 @@ class ClientAuthenticationMethodValidatingRequestEntityConverter<T extends Abstr
 				|| clientAuthenticationMethod.equals(ClientAuthenticationMethod.CLIENT_SECRET_POST);
 		Assert.isTrue(supportedClientAuthenticationMethod, () -> String.format(
 				"This class supports `client_secret_basic`, `client_secret_post`, and `none` by default. Client [%s] is using [%s] instead. Please use a supported client authentication method, or use `setRequestEntityConverter` to supply an instance that supports [%s].",
-				registrationId, clientAuthenticationMethod, clientAuthenticationMethod));
+				registrationId, clientAuthenticationMethod.getValue(), clientAuthenticationMethod.getValue()));
 		return this.delegate.convert(grantRequest);
 	}