diff --git a/common/sealable-trie/src/nodes.rs b/common/sealable-trie/src/nodes.rs
index 29a09069..e714f9fd 100644
--- a/common/sealable-trie/src/nodes.rs
+++ b/common/sealable-trie/src/nodes.rs
@@ -208,11 +208,13 @@ impl<'a, P, S> Node<'a, P, S> {
                 // tag = 0b100v_0000 where v indicates whether the child is
                 // a value reference.
                 let tag = 0x80 | (u8::from(child.is_value()) << 4);
-                // XXX
                 if let Ok(len) = key.encode_into(key_buf, tag) {
                     buf[len..len + 32].copy_from_slice(child.hash().as_slice());
                     len + 32
                 } else {
+                    // We support Extension nodes with invalid keys so this
+                    // method is infallible.  It’s easier to handle it than have
+                    // callers deal with errors.
                     return hash_extension_slow_path(*key, child);
                 }
             }
diff --git a/common/sealable-trie/src/nodes/tests.rs b/common/sealable-trie/src/nodes/tests.rs
index 99b750da..d9ea38e4 100644
--- a/common/sealable-trie/src/nodes/tests.rs
+++ b/common/sealable-trie/src/nodes/tests.rs
@@ -1,5 +1,3 @@
-use base64::engine::general_purpose::STANDARD as BASE64_ENGINE;
-use base64::Engine;
 use lib::hash::CryptoHash;
 use memory::Ptr;
 use pretty_assertions::assert_eq;
@@ -48,17 +46,24 @@ fn check_node_encoding(node: Node, want: [u8; RawNode::SIZE], want_hash: &str) {
     assert_eq!(want, raw.0, "Unexpected raw representation");
     assert_eq!(node, RawNode(want).decode(), "Bad Raw→Node conversion");
 
-    let want_hash = BASE64_ENGINE.decode(want_hash).unwrap();
-    let want_hash = <&[u8; 32]>::try_from(want_hash.as_slice()).unwrap();
-    let want_hash = CryptoHash::from(*want_hash);
+    let want_hash = b64decode(want_hash);
     assert_eq!(want_hash, node.hash(), "Unexpected hash of {node:?}");
-
     if let Node::Extension { key, child } = node {
         let got = super::hash_extension_slow_path(key, &child);
         assert_eq!(want_hash, got, "Unexpected slow path hash of {node:?}");
     }
 }
 
+/// Decodes base64-encoded CryptoHash; panics on error.
+fn b64decode(hash: &str) -> CryptoHash {
+    use base64::engine::general_purpose::STANDARD as BASE64_ENGINE;
+    use base64::Engine;
+
+    let hash = BASE64_ENGINE.decode(hash).unwrap();
+    let hash = <&[u8; 32]>::try_from(hash.as_slice()).unwrap();
+    CryptoHash::from(*hash)
+}
+
 #[test]
 #[rustfmt::skip]
 fn test_branch_encoding() {
@@ -214,6 +219,32 @@ fn test_extension_encoding() {
     ], "uU9GlH+fEQAnezn3HWuvo/ZSBIhuSkuE2IGjhUFdC04=");
 }
 
+#[test]
+fn test_extension_hash_bad_key() {
+    let empty_key = bits::Slice::new(&[], 0, 0).unwrap();
+    let long_key = bits::Slice::new(&[0; 53], 2, 420).unwrap();
+    for (child, sealed, if_empty, if_long) in [
+        (
+            Reference::node(Some(DEAD), &ONE),
+            Reference::node(None, &ONE),
+            b64decode("NXOo9QBg+AbJSM/zh4Rikg8R5otOByKUJfiWhKUiZ5Y="),
+            b64decode("Q/Er5mIa2gsawPOfF+Q/2XO5l29WZyknw/kyI53tGXo="),
+        ),
+        (
+            Reference::value(false, &ONE),
+            Reference::value(true, &ONE),
+            b64decode("LtVLGf1mBecG3z2Lcq1IXOGo/zQpGxWbXN977zUZMpI="),
+            b64decode("BMi90/Oois7h3CPNz6y8BB/9agz2mkAePLFQt2hdluM="),
+        ),
+    ] {
+        assert_eq!(if_empty, Node::Extension { key: empty_key, child }.hash());
+        assert_eq!(if_long, Node::Extension { key: long_key, child }.hash());
+
+        assert_eq!(if_empty, Node::Extension { key: empty_key, child: sealed }.hash());
+        assert_eq!(if_long, Node::Extension { key: long_key, child: sealed }.hash());
+    }
+}
+
 #[test]
 #[rustfmt::skip]
 fn test_value_encoding() {