diff --git a/compose_viz/service.py b/compose_viz/service.py
index f1da6a8..feaaf53 100644
--- a/compose_viz/service.py
+++ b/compose_viz/service.py
@@ -10,6 +10,9 @@ class Service:
         if image is None and extends is None:
             raise ValueError(f"Both image and extends are not defined in service '{name}', aborting.")
 
+        if image is not None and extends is not None:
+            raise ValueError(f"Only one of image and extends can be defined in service '{name}', aborting.")
+
         self._image = image
         self._ports = ports
         self._networks = networks