diff --git a/compiler/mono/src/ir.rs b/compiler/mono/src/ir.rs
index a1f595fcf0..364dafcbdf 100644
--- a/compiler/mono/src/ir.rs
+++ b/compiler/mono/src/ir.rs
@@ -4131,17 +4131,17 @@ fn convert_tag_union<'a>(
             hole,
         ),
         ByteUnion(tag_names) => {
-            let tag_id = tag_names
-                .iter()
-                .position(|key| key == &tag_name)
-                .expect("tag must be in its own type");
+            let opt_tag_id = tag_names.iter().position(|key| key == &tag_name);
 
-            Stmt::Let(
-                assigned,
-                Expr::Literal(Literal::Byte(tag_id as u8)),
-                Layout::Builtin(Builtin::Int8),
-                hole,
-            )
+            match opt_tag_id {
+                Some(tag_id) => Stmt::Let(
+                    assigned,
+                    Expr::Literal(Literal::Byte(tag_id as u8)),
+                    Layout::Builtin(Builtin::Int8),
+                    hole,
+                ),
+                None => Stmt::RuntimeError("tag must be in its own type"),
+            }
         }
 
         Newtype {
diff --git a/compiler/test_gen/src/gen_tags.rs b/compiler/test_gen/src/gen_tags.rs
index 776c06ff07..cfd11b807f 100644
--- a/compiler/test_gen/src/gen_tags.rs
+++ b/compiler/test_gen/src/gen_tags.rs
@@ -1033,3 +1033,20 @@ fn applied_tag_function_linked_list() {
         i64
     );
 }
+
+#[test]
+#[should_panic(expected = "")]
+fn tag_must_be_its_own_type() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            z : [ A, B, C ]
+            z = Z
+
+            z
+            "#
+        ),
+        1,
+        i64
+    );
+}