diff --git a/scripts/tinyhouse/__main__.py b/scripts/tinyhouse/__main__.py
index c88ed71..e3d4116 100644
--- a/scripts/tinyhouse/__main__.py
+++ b/scripts/tinyhouse/__main__.py
@@ -110,6 +110,7 @@ def check(flows: list[Flow]) -> list[str]:
     problems += _check_widgets(known)
     problems += _check_sources(flows)
     problems += _check_schemas(flows)
+    problems += _check_injects(flows)
     return problems
 
 
@@ -308,6 +309,36 @@ def _rejects(spec: dict[str, Any], sample: Any) -> str:
     return ""
 
 
+def _check_injects(flows: list[Flow]) -> list[str]:
+    """What an inject emits has to be what its port declared.
+
+    An inject left to itself emits the current time, so a port expecting
+    anything but a number gets a float and the node raises — five times, and
+    the supervisor quarantines the flow. Nothing else catches it: an inject is
+    not a Python node, so it has no source to run, and both sides of the
+    declaration agree with each other while disagreeing with reality.
+    """
+    problems = []
+    for flow in flows:
+        for node in flow.nodes:
+            if node["type"] != "inject":
+                continue
+            params = node.get("params", {})
+            payloads = dict(params.get("payloads") or {})
+            for spec in node.get("provides", []):
+                port = spec.get("port") or spec["name"]
+                if port in payloads:
+                    value = payloads[port]
+                elif "payload" in params:
+                    value = params["payload"]
+                else:
+                    value = 0.0  # the current time, which is what it defaults to
+                bad = _rejects(spec, value)
+                if bad:
+                    problems.append(f"{flow.name}.{node['id']} emits: {bad}")
+    return problems
+
+
 def _check_schemas(flows: list[Flow]) -> list[str]:
     """Hand every node to the engine's own models before the API sees them.
 
diff --git a/scripts/tinyhouse/sensing.py b/scripts/tinyhouse/sensing.py
index 8bb9e79..3503577 100644
--- a/scripts/tinyhouse/sensing.py
+++ b/scripts/tinyhouse/sensing.py
@@ -165,7 +165,10 @@ def power(h: dict[str, Any]) -> Flow:
             "id": "keepalive_tick",
             "type": "inject",
             "title": "Every 30 seconds",
-            "params": {"interval": 30, "at_start": True},
+            # An empty payload, as the reference sends: the Cerbo only wants
+            # to know somebody is listening. Left to itself an inject emits
+            # the time, which is a float going into a string port.
+            "params": {"interval": 30, "at_start": True, "payload": ""},
             "provides": [{"name": "keepalive", "dtype": "str"}],
         }
     )