diff --git a/extensions/grpc/protoc/src/main/java/io/quarkus/grpc/protoc/plugin/MutinyGrpcGenerator.java b/extensions/grpc/protoc/src/main/java/io/quarkus/grpc/protoc/plugin/MutinyGrpcGenerator.java
index 65743431321b81..308b8657967fc9 100644
--- a/extensions/grpc/protoc/src/main/java/io/quarkus/grpc/protoc/plugin/MutinyGrpcGenerator.java
+++ b/extensions/grpc/protoc/src/main/java/io/quarkus/grpc/protoc/plugin/MutinyGrpcGenerator.java
@@ -3,6 +3,8 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.logging.Level;
+import java.util.logging.Logger;
 import java.util.stream.Collectors;
 
 import com.google.common.base.Strings;
@@ -19,6 +21,8 @@
  */
 public class MutinyGrpcGenerator extends Generator {
 
+    private static final Logger log = Logger.getLogger(MutinyGrpcGenerator.class.getName());
+
     private static final int SERVICE_NUMBER_OF_PATHS = 2;
     private static final int METHOD_NUMBER_OF_PATHS = 4;
     public static final String CLASS_PREFIX = "Mutiny";
@@ -41,9 +45,25 @@ public List<PluginProtos.CodeGeneratorResponse.File> generateFiles(PluginProtos.
                 .collect(Collectors.toList());
 
         List<ServiceContext> services = findServices(protosToGenerate, typeMap);
+        validateServices(services);
         return generateFiles(services);
     }
 
+    private void validateServices(List<ServiceContext> services) {
+        boolean failed = false;
+        for (ServiceContext service : services) {
+            if (service.packageName == null || service.packageName.isBlank()) {
+                log.log(Level.SEVERE, "Using the default java package is not supported for "
+                        + "Quarkus RPC code generation. Please specify `option java_package = \"your.package\"` in "
+                        + service.protoName);
+                failed = true;
+            }
+        }
+        if (failed) {
+            throw new IllegalArgumentException("Code generation failed. Please check the log above for details.");
+        }
+    }
+
     private List<ServiceContext> findServices(List<DescriptorProtos.FileDescriptorProto> protos, ProtoTypeMap typeMap) {
         List<ServiceContext> contexts = new ArrayList<>();