don't allow DocumentFragments as the morph target

index.js

@@ -33,6 +33,12 @@ function nanomorph (oldTree, newTree) {
   // }
   assert.equal(typeof oldTree, 'object', 'nanomorph: oldTree should be an object')
   assert.equal(typeof newTree, 'object', 'nanomorph: newTree should be an object')
+  assert.notEqual(
+    newTree.nodeType,
+    11,
+    'nanomorph: newTree should have one root node (which is not a DocumentFragment)'
+  )
+
   var tree = walk(newTree, oldTree)
   // if (DEBUG) console.log('=> morphed\n  %s', tree.outerHTML)
   return tree

test/diff.js

@@ -440,3 +440,11 @@ tape('use id as a key hint', function (t) {
     t.end()
   })
 })
+
+tape('disallow document fragments', function (t) {
+  var a = html`<div><div>a</div></div>`
+  var b = html`<div>a</div><div>b</div>`
+
+  t.throws(nanomorph.bind(null, a, b), /newTree should have one root node/, 'no fragments')
+  t.end()
+})