diff --git a/src/elements/logic/gate.typ b/src/elements/logic/gate.typ
index 825adbd..bd0fb23 100644
--- a/src/elements/logic/gate.typ
+++ b/src/elements/logic/gate.typ
@@ -63,6 +63,12 @@
     if to == "out" {
       dy = height / 2
     } else {
+      let all-ports = range(inputs).map(i => "in" + str(i))
+      let i = all-ports.position(p => p == to)
+      assert(
+        i != none,
+        message: "Unknown port '" + y.to + "' must be one of " + repr(all-ports)
+      )
       dy = height * (i + 0.5) / inputs
     }