diff --git a/src/circuit.typ b/src/circuit.typ
index bdf6184..2af8565 100644
--- a/src/circuit.typ
+++ b/src/circuit.typ
@@ -11,26 +11,40 @@
 #let circuit(body, length: 2em) = {
   let next-id = 0
   let elements = (:)
+  let ids = ()
+  for e in body {
+    if type(e) == dictionary and "id" in e {
+      ids.push(e.id)
+    }
+  }
 
   for element in body {
     let internal = type(element) == dictionary and "id" in element
-    let eid = if internal {element.id} else {none}
-    if eid == none {
-      while str(next-id) in elements {
+    let eid = if internal {element.id} else {auto}
+    if eid == auto {
+      while str(next-id) in ids {
         next-id += 1
       }
       eid = str(next-id)
+      ids.push(eid)
       if internal {
         element.id = eid
       }
       next-id += 1
     }
+    if eid in elements {
+      panic("An element with the id '" + eid + "' already exists. Please use a different id")
+    }
     elements.insert(eid, element)
   }
 
   for element in elements.values() {
     if type(element) == dictionary and "pre-process" in element {
       elements = (element.pre-process)(elements, element)
+      assert(
+        type(elements) == dictionary,
+        message: "The `pre-process` method of element '" + element.id + "' did not return the elements dictionary"
+      )
     }
   }
 
diff --git a/src/elements/element.typ b/src/elements/element.typ
index 2dc0bb6..d83c4b5 100644
--- a/src/elements/element.typ
+++ b/src/elements/element.typ
@@ -147,7 +147,19 @@
 
   // Workaround because CeTZ needs to have all draw functions in the body
   let func = {}
-  (func, bounds) = draw-shape(elmt, bounds)
+  let res = draw-shape(elmt, bounds)
+  assert(
+    type(res) == array and res.len() == 2,
+    message: "The drawing function of element '" + elmt.id + "' did not return a function and new bounds"
+  )
+  (func, bounds) = res
+  if type(func) == function {
+    func = (func,)
+  }
+  assert(
+    type(bounds) == dictionary,
+    message: "The drawing function of element '" + elmt.id + "' did not return the correct bounds dictionary"
+  )
   func
 
   if elmt.name != none {
@@ -208,7 +220,7 @@
   ports-margins: (:),
   fill: none,
   stroke: black + 1pt,
-  id: "",
+  id: auto,
   auto-ports: true,
   ports-y: (:),
   debug: (