diff --git a/gravitee-am-gateway/gravitee-am-gateway-handler/gravitee-am-gateway-handler-oidc/src/main/java/io/gravitee/am/gateway/handler/oauth2/service/granter/extensiongrant/ExtensionGrantGranter.java b/gravitee-am-gateway/gravitee-am-gateway-handler/gravitee-am-gateway-handler-oidc/src/main/java/io/gravitee/am/gateway/handler/oauth2/service/granter/extensiongrant/ExtensionGrantGranter.java
index 2ab77848c36..60432320045 100644
--- a/gravitee-am-gateway/gravitee-am-gateway-handler/gravitee-am-gateway-handler-oidc/src/main/java/io/gravitee/am/gateway/handler/oauth2/service/granter/extensiongrant/ExtensionGrantGranter.java
+++ b/gravitee-am-gateway/gravitee-am-gateway-handler/gravitee-am-gateway-handler-oidc/src/main/java/io/gravitee/am/gateway/handler/oauth2/service/granter/extensiongrant/ExtensionGrantGranter.java
@@ -38,6 +38,7 @@
 import io.gravitee.am.model.oidc.Client;
 import io.reactivex.rxjava3.core.Maybe;
 import io.reactivex.rxjava3.core.Single;
+import org.apache.commons.lang3.StringUtils;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -160,7 +161,8 @@ protected Maybe<User> resolveResourceOwner(TokenRequest tokenRequest, Client cli
                     }
                 })
                 .onErrorResumeNext(ex -> {
-                    return Maybe.error(new InvalidGrantException(ex.getMessage()));
+                    String msg = StringUtils.isBlank(ex.getMessage()) ? "Unknown error" : ex.getMessage();
+                    return Maybe.error(new InvalidGrantException(msg));
                 });
     }
 
diff --git a/gravitee-am-gateway/gravitee-am-gateway-handler/gravitee-am-gateway-handler-oidc/src/test/java/io/gravitee/am/gateway/handler/oauth2/service/granter/extensiongrant/ExtensionGrantGranterTest.java b/gravitee-am-gateway/gravitee-am-gateway-handler/gravitee-am-gateway-handler-oidc/src/test/java/io/gravitee/am/gateway/handler/oauth2/service/granter/extensiongrant/ExtensionGrantGranterTest.java
new file mode 100644
index 00000000000..bc452d75e27
--- /dev/null
+++ b/gravitee-am-gateway/gravitee-am-gateway-handler/gravitee-am-gateway-handler-oidc/src/test/java/io/gravitee/am/gateway/handler/oauth2/service/granter/extensiongrant/ExtensionGrantGranterTest.java
@@ -0,0 +1,88 @@
+/**
+ * Copyright (C) 2015 The Gravitee team (http://gravitee.io)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.gravitee.am.gateway.handler.oauth2.service.granter.extensiongrant;
+
+import io.gravitee.am.extensiongrant.api.ExtensionGrantProvider;
+import io.gravitee.am.gateway.handler.common.vertx.core.http.VertxHttpHeaders;
+import io.gravitee.am.gateway.handler.oauth2.service.request.TokenRequest;
+import io.gravitee.am.model.ExtensionGrant;
+import io.gravitee.am.model.oidc.Client;
+import io.reactivex.rxjava3.core.Maybe;
+import io.vertx.core.http.impl.headers.HeadersMultiMap;
+import org.junit.Test;
+import org.mockito.Mockito;
+
+
+public class ExtensionGrantGranterTest {
+
+    @Test
+    public void when_ex_msg_is_missing_present_default_one(){
+        ExtensionGrantProvider extensionGrantProvider = Mockito.mock();
+        ExtensionGrant extensionGrant = new ExtensionGrant();
+        extensionGrant.setGrantType("jwt-bearer");
+
+        ExtensionGrantGranter granter = new ExtensionGrantGranter(
+                extensionGrantProvider,
+                extensionGrant,
+                Mockito.mock(),
+                Mockito.mock(),
+                Mockito.mock(),
+                Mockito.mock(),
+                Mockito.mock(),
+                Mockito.mock());
+
+        TokenRequest tokenRequest = new TokenRequest();
+        tokenRequest.setParameters(new VertxHttpHeaders(new HeadersMultiMap()));
+        Client client = new Client();
+
+        Mockito.when(extensionGrantProvider.grant(Mockito.any())).thenReturn(Maybe.error(new RuntimeException("")));
+
+        granter.resolveResourceOwner(tokenRequest, client)
+                .test()
+                .assertError(ex -> ex.getMessage().equals("Unknown error"));
+
+    }
+
+    @Test
+    public void when_ex_msg_is_present_return_it(){
+        ExtensionGrantProvider extensionGrantProvider = Mockito.mock();
+        ExtensionGrant extensionGrant = new ExtensionGrant();
+        extensionGrant.setGrantType("jwt-bearer");
+
+        ExtensionGrantGranter granter = new ExtensionGrantGranter(
+                extensionGrantProvider,
+                extensionGrant,
+                Mockito.mock(),
+                Mockito.mock(),
+                Mockito.mock(),
+                Mockito.mock(),
+                Mockito.mock(),
+                Mockito.mock());
+
+        TokenRequest tokenRequest = new TokenRequest();
+        tokenRequest.setParameters(new VertxHttpHeaders(new HeadersMultiMap()));
+        Client client = new Client();
+
+        Mockito.when(extensionGrantProvider.grant(Mockito.any())).thenReturn(Maybe.error(new RuntimeException("message")));
+
+        granter.resolveResourceOwner(tokenRequest, client)
+                .test()
+                .assertError(ex -> ex.getMessage().equals("message"));
+
+    }
+
+
+}
\ No newline at end of file