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 65743431321b8..fc11f55503d3e 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
@@ -41,9 +41,25 @@ 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()) {
+                System.err.println("Using the default java package is not supported for "
+                        + "gRPC 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<>();