diff --git a/services/src/main/java/org/keycloak/protocol/oidc/endpoints/TokenEndpoint.java b/services/src/main/java/org/keycloak/protocol/oidc/endpoints/TokenEndpoint.java
index 4505c9fb51..adb73ff47a 100644
--- a/services/src/main/java/org/keycloak/protocol/oidc/endpoints/TokenEndpoint.java
+++ b/services/src/main/java/org/keycloak/protocol/oidc/endpoints/TokenEndpoint.java
@@ -266,7 +266,8 @@ public class TokenEndpoint {
             event.event(EventType.PERMISSION_TOKEN);
             action = Action.PERMISSION;
         } else {
-            throw new CorsErrorResponseException(cors, Errors.INVALID_REQUEST, "Invalid " + OIDCLoginProtocol.GRANT_TYPE_PARAM, Response.Status.BAD_REQUEST);
+            throw new CorsErrorResponseException(cors, OAuthErrorException.UNSUPPORTED_GRANT_TYPE,
+                "Unsupported " + OIDCLoginProtocol.GRANT_TYPE_PARAM, Response.Status.BAD_REQUEST);
         }
 
         event.detail(Details.GRANT_TYPE, grantType);
diff --git a/testsuite/integration-arquillian/tests/base/src/test/java/org/keycloak/testsuite/oauth/ResourceOwnerPasswordCredentialsGrantTest.java b/testsuite/integration-arquillian/tests/base/src/test/java/org/keycloak/testsuite/oauth/ResourceOwnerPasswordCredentialsGrantTest.java
index 8f5f9905bc..b9c56916d2 100755
--- a/testsuite/integration-arquillian/tests/base/src/test/java/org/keycloak/testsuite/oauth/ResourceOwnerPasswordCredentialsGrantTest.java
+++ b/testsuite/integration-arquillian/tests/base/src/test/java/org/keycloak/testsuite/oauth/ResourceOwnerPasswordCredentialsGrantTest.java
@@ -18,9 +18,12 @@
 package org.keycloak.testsuite.oauth;
 
 import org.apache.http.HttpResponse;
+import org.apache.http.NameValuePair;
+import org.apache.http.client.entity.UrlEncodedFormEntity;
 import org.apache.http.client.methods.HttpPost;
 import org.apache.http.impl.client.CloseableHttpClient;
 import org.apache.http.impl.client.HttpClientBuilder;
+import org.apache.http.message.BasicNameValuePair;
 import org.junit.Rule;
 import org.junit.Test;
 import org.keycloak.OAuth2Constants;
@@ -53,6 +56,8 @@ import org.keycloak.testsuite.util.TokenSignatureUtil;
 import org.keycloak.testsuite.util.UserBuilder;
 import org.keycloak.testsuite.util.UserManager;
 
+import java.io.UnsupportedEncodingException;
+import java.util.LinkedList;
 import java.util.List;
 
 import static org.junit.Assert.assertEquals;
@@ -587,4 +592,27 @@ public class ResourceOwnerPasswordCredentialsGrantTest extends AbstractKeycloakT
         }
     }
 
+    @Test
+    public void grantAccessTokenUnsupportedGrantType() throws Exception {
+        oauth.clientId("resource-owner");
+
+        try (CloseableHttpClient client = HttpClientBuilder.create().build()) {
+            HttpPost post = new HttpPost(oauth.getResourceOwnerPasswordCredentialGrantUrl());
+            List<NameValuePair> parameters = new LinkedList<>();
+            parameters.add(new BasicNameValuePair(OAuth2Constants.GRANT_TYPE, "unsupported_grant_type"));
+            UrlEncodedFormEntity formEntity;
+            try {
+                formEntity = new UrlEncodedFormEntity(parameters, "UTF-8");
+            } catch (UnsupportedEncodingException e) {
+                throw new RuntimeException(e);
+            }
+            post.setEntity(formEntity);
+            OAuthClient.AccessTokenResponse response = new OAuthClient.AccessTokenResponse(client.execute(post));
+
+            assertEquals(400, response.getStatusCode());
+
+            assertEquals(OAuthErrorException.UNSUPPORTED_GRANT_TYPE, response.getError());
+            assertEquals("Unsupported grant_type", response.getErrorDescription());
+        }
+    }
 }