# DEVELOPMENT FIXTURE — illustrative synthetic patch; not safe to ship.
diff --git a/src/forecast-tool.ts b/src/forecast-tool.ts
--- a/src/forecast-tool.ts
+++ b/src/forecast-tool.ts
@@
-  return { temperatureC: Number(payload.temperature_c) };
+  if (typeof payload.temperature_c !== "number") throw new TypeError("temperature_c");
+  return { temperatureC: payload.temperature_c };