# must fail: union_O9
# the union is the floor, the intersection (codecs::union_of)
diff --git a/wai/wai-rs/src/http_binding/codecs.rs b/wai/wai-rs/src/http_binding/codecs.rs
index 530ae822..97e32ca4 100644
--- a/wai/wai-rs/src/http_binding/codecs.rs
+++ b/wai/wai-rs/src/http_binding/codecs.rs
@@ -338,8 +338,9 @@ pub fn union_of(objects: &[SeqObject]) -> Result<Element, CodecsError> {
     let each = described(objects)?;
     let prefix = objects.iter().map(|o| if o.head.version == Version::Wai2 { Prefix::Wai2 } else { Prefix::Wai1 }).max().unwrap_or(Prefix::Wai1);
     let mut alts: Vec<Alt> = Vec::new();
+    let all = each.clone();
     for e in each {
-        for a in e.alts {
+        for a in e.alts.into_iter().filter(|a| all.iter().all(|o| o.alts.iter().any(|x| x.cap == a.cap))) {
             match alts.iter_mut().find(|x| x.cap == a.cap) {
                 Some(x) => {
                     for p in a.plus {