diff --git a/engine/runtime-instrument-common/src/main/java/org/enso/interpreter/service/ExecutionProgressObserver.java b/engine/runtime-instrument-common/src/main/java/org/enso/interpreter/service/ExecutionProgressObserver.java
index 64c1be87663f..de515bfc0a67 100644
--- a/engine/runtime-instrument-common/src/main/java/org/enso/interpreter/service/ExecutionProgressObserver.java
+++ b/engine/runtime-instrument-common/src/main/java/org/enso/interpreter/service/ExecutionProgressObserver.java
@@ -49,8 +49,14 @@ public void accept(ObservedMessage t) {
             aggregate.advanceBy(key, by.longValue());
           }
         }
+        case "LOG {}:{}" -> {
+          if (t.getArguments().size() >= 2 && t.getArguments().get(1) instanceof String msg) {
+            var key = t.getArguments().get(0);
+            // TBD: now what?
+          }
+        }
         default -> {
-          System.err.println("  seeing " + t.getMessage() + " for " + nodeId);
+          assert false : "Unexpected progress message: " + t.getMessage();
         }
       }
     }