diff --git a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientAuthorizationCodeTokenResponseClientTests.java b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientAuthorizationCodeTokenResponseClientTests.java
index a7a4acb982..3b65361f02 100644
--- a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientAuthorizationCodeTokenResponseClientTests.java
+++ b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientAuthorizationCodeTokenResponseClientTests.java
@@ -364,6 +364,7 @@ public class RestClientAuthorizationCodeTokenResponseClientTests {
 				this.authorizationExchange);
 		Converter<OAuth2AuthorizationCodeGrantRequest, HttpHeaders> headersConverter = mock();
 		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
 		headers.put("custom-header-name", Collections.singletonList("custom-header-value"));
 		given(headersConverter.convert(grantRequest)).willReturn(headers);
 		this.tokenResponseClient.setHeadersConverter(headersConverter);
diff --git a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientClientCredentialsTokenResponseClientTests.java b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientClientCredentialsTokenResponseClientTests.java
index 77f8adba69..4cfa4c7205 100644
--- a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientClientCredentialsTokenResponseClientTests.java
+++ b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientClientCredentialsTokenResponseClientTests.java
@@ -366,6 +366,7 @@ public class RestClientClientCredentialsTokenResponseClientTests {
 		OAuth2ClientCredentialsGrantRequest grantRequest = new OAuth2ClientCredentialsGrantRequest(clientRegistration);
 		Converter<OAuth2ClientCredentialsGrantRequest, HttpHeaders> headersConverter = mock();
 		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
 		headers.put("custom-header-name", Collections.singletonList("custom-header-value"));
 		given(headersConverter.convert(grantRequest)).willReturn(headers);
 		this.tokenResponseClient.setHeadersConverter(headersConverter);
diff --git a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientJwtBearerTokenResponseClientTests.java b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientJwtBearerTokenResponseClientTests.java
index 79fb9aee02..0d5d3a37c5 100644
--- a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientJwtBearerTokenResponseClientTests.java
+++ b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientJwtBearerTokenResponseClientTests.java
@@ -339,6 +339,7 @@ public class RestClientJwtBearerTokenResponseClientTests {
 		JwtBearerGrantRequest grantRequest = new JwtBearerGrantRequest(clientRegistration, this.jwtAssertion);
 		Converter<JwtBearerGrantRequest, HttpHeaders> headersConverter = mock();
 		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
 		headers.put("custom-header-name", Collections.singletonList("custom-header-value"));
 		given(headersConverter.convert(grantRequest)).willReturn(headers);
 		this.tokenResponseClient.setHeadersConverter(headersConverter);
diff --git a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientRefreshTokenTokenResponseClientTests.java b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientRefreshTokenTokenResponseClientTests.java
index ea0f883b56..bad6b3fa4f 100644
--- a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientRefreshTokenTokenResponseClientTests.java
+++ b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientRefreshTokenTokenResponseClientTests.java
@@ -384,6 +384,7 @@ public class RestClientRefreshTokenTokenResponseClientTests {
 				this.accessToken, this.refreshToken);
 		Converter<OAuth2RefreshTokenGrantRequest, HttpHeaders> headersConverter = mock();
 		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
 		headers.put("custom-header-name", Collections.singletonList("custom-header-value"));
 		given(headersConverter.convert(grantRequest)).willReturn(headers);
 		this.tokenResponseClient.setHeadersConverter(headersConverter);
diff --git a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientTokenExchangeTokenResponseClientTests.java b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientTokenExchangeTokenResponseClientTests.java
index c81f767677..8d2f645490 100644
--- a/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientTokenExchangeTokenResponseClientTests.java
+++ b/oauth2/oauth2-client/src/test/java/org/springframework/security/oauth2/client/endpoint/RestClientTokenExchangeTokenResponseClientTests.java
@@ -465,6 +465,7 @@ public class RestClientTokenExchangeTokenResponseClientTests {
 				this.actorToken);
 		Converter<TokenExchangeGrantRequest, HttpHeaders> headersConverter = mock();
 		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
 		headers.put("custom-header-name", Collections.singletonList("custom-header-value"));
 		given(headersConverter.convert(grantRequest)).willReturn(headers);
 		this.tokenResponseClient.setHeadersConverter(headersConverter);