diff --git a/vertx-auth-oauth2/src/main/java/io/vertx/ext/auth/oauth2/providers/OpenIDConnectAuth.java b/vertx-auth-oauth2/src/main/java/io/vertx/ext/auth/oauth2/providers/OpenIDConnectAuth.java
index f50474386..e7380028f 100644
--- a/vertx-auth-oauth2/src/main/java/io/vertx/ext/auth/oauth2/providers/OpenIDConnectAuth.java
+++ b/vertx-auth-oauth2/src/main/java/io/vertx/ext/auth/oauth2/providers/OpenIDConnectAuth.java
@@ -135,11 +135,10 @@ static Future<OAuth2Auth> discover(final Vertx vertx, final OAuth2Options config
         }
 
 
-        // reset config
-        config.setSupportedGrantTypes(null);
-
         if (json.containsKey("grant_types_supported")) {
-          // optional config
+          // optional config, only replace the configured grant types when the server
+          // announces the ones it supports
+          config.setSupportedGrantTypes(null);
           JsonArray flows = json.getJsonArray("grant_types_supported");
           flows.forEach(el -> config.addSupportedGrantType((String) el));
         }
diff --git a/vertx-auth-oauth2/src/test/java/io/vertx/tests/OpenIDCDiscoveryGrantTypesTest.java b/vertx-auth-oauth2/src/test/java/io/vertx/tests/OpenIDCDiscoveryGrantTypesTest.java
new file mode 100644
index 000000000..f74c62501
--- /dev/null
+++ b/vertx-auth-oauth2/src/test/java/io/vertx/tests/OpenIDCDiscoveryGrantTypesTest.java
@@ -0,0 +1,129 @@
+package io.vertx.tests;
+
+import io.vertx.core.http.HttpMethod;
+import io.vertx.core.http.HttpServer;
+import io.vertx.core.json.JsonArray;
+import io.vertx.core.json.JsonObject;
+import io.vertx.ext.auth.oauth2.OAuth2Options;
+import io.vertx.ext.auth.oauth2.impl.OAuth2AuthProviderImpl;
+import io.vertx.ext.auth.oauth2.providers.OpenIDConnectAuth;
+import io.vertx.ext.unit.Async;
+import io.vertx.ext.unit.TestContext;
+import io.vertx.ext.unit.junit.RunTestOnContext;
+import io.vertx.ext.unit.junit.VertxUnitRunner;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import java.util.Arrays;
+import java.util.Collections;
+
+@RunWith(VertxUnitRunner.class)
+public class OpenIDCDiscoveryGrantTypesTest {
+
+  @Rule
+  public final RunTestOnContext rule = new RunTestOnContext();
+
+  private HttpServer server;
+  private String site;
+  private JsonObject discoveryDocument;
+
+  @Before
+  public void setUp(TestContext should) throws Exception {
+    final Async setup = should.async();
+
+    server = rule.vertx().createHttpServer().requestHandler(req -> {
+      if (req.method() == HttpMethod.GET && "/.well-known/openid-configuration".equals(req.path())) {
+        req.response()
+          .putHeader("Content-Type", "application/json")
+          .end(discoveryDocument.encode());
+      } else {
+        req.response().setStatusCode(404).end();
+      }
+    });
+    server.listen(0).onComplete(ready -> {
+      if (ready.failed()) {
+        throw new RuntimeException(ready.cause());
+      }
+      site = "http://localhost:" + ready.result().actualPort();
+      // ready
+      setup.complete();
+    });
+  }
+
+  @After
+  public void tearDown(TestContext should) throws Exception {
+    final Async tearDown = should.async();
+    server.close()
+      .onFailure(should::fail)
+      .onSuccess(v -> tearDown.complete());
+  }
+
+  private JsonObject baseDiscoveryDocument() {
+    return new JsonObject()
+      .put("issuer", site)
+      .put("authorization_endpoint", site + "/oauth/authorize")
+      .put("token_endpoint", site + "/oauth/token");
+  }
+
+  @Test
+  public void configuredGrantTypesSurviveWhenServerOmitsThem(TestContext should) {
+    final Async test = should.async();
+
+    discoveryDocument = baseDiscoveryDocument();
+
+    // a client used only to validate tokens, no clientId is required for the implicit flow
+    OpenIDConnectAuth.discover(rule.vertx(), new OAuth2Options()
+        .setSite(site)
+        .setSupportedGrantTypes(Collections.singletonList("implicit")))
+      .onComplete(discovery -> {
+        if (discovery.failed()) {
+          should.fail(discovery.cause().getMessage());
+        } else {
+          OAuth2Options config = ((OAuth2AuthProviderImpl) discovery.result()).getConfig();
+          should.assertEquals(Collections.singletonList("implicit"), config.getSupportedGrantTypes());
+          test.complete();
+        }
+      });
+  }
+
+  @Test
+  public void serverGrantTypesReplaceConfiguredOnes(TestContext should) {
+    final Async test = should.async();
+
+    discoveryDocument = baseDiscoveryDocument()
+      .put("grant_types_supported", new JsonArray().add("authorization_code").add("refresh_token"));
+
+    OpenIDConnectAuth.discover(rule.vertx(), new OAuth2Options()
+        .setSite(site)
+        .setClientId("client-id")
+        .setSupportedGrantTypes(Collections.singletonList("implicit")))
+      .onComplete(discovery -> {
+        if (discovery.failed()) {
+          should.fail(discovery.cause().getMessage());
+        } else {
+          OAuth2Options config = ((OAuth2AuthProviderImpl) discovery.result()).getConfig();
+          should.assertEquals(Arrays.asList("authorization_code", "refresh_token"), config.getSupportedGrantTypes());
+          test.complete();
+        }
+      });
+  }
+
+  @Test
+  public void missingGrantTypesStillRequireClientIdByDefault(TestContext should) {
+    final Async test = should.async();
+
+    discoveryDocument = baseDiscoveryDocument();
+
+    // no grant types configured and none discovered, the defaults require a clientId
+    OpenIDConnectAuth.discover(rule.vertx(), new OAuth2Options()
+        .setSite(site))
+      .onComplete(discovery -> {
+        should.assertTrue(discovery.failed());
+        should.assertEquals("Configuration missing. You need to specify [clientId]", discovery.cause().getMessage());
+        test.complete();
+      });
+  }
+}