diff --git a/packages/html2/src/html.ts b/packages/html2/src/html.ts
index 4bf1c81..a324512 100644
--- a/packages/html2/src/html.ts
+++ b/packages/html2/src/html.ts
@@ -70,7 +70,22 @@ function nodeInserter(n: number): PlaceholderAction {
                 switch (typeof f) {
                     case 'string': newNode = document.createTextNode(f); break;
                     case 'number': newNode = document.createTextNode('' + f); break;
-                    default: newNode = f; break;
+                    case 'object':
+                        if (f !== null && 'nodeType' in f) {
+                            newNode = f;
+                            break;
+                        }
+                        /* fall through */
+                    default: {
+                        let info;
+                        try {
+                            info = '' + f;
+                        } catch (_e) {
+                            info = (f as any).toString();
+                        }
+                        newNode = document.createTextNode(`<ERROR: invalid HtmlFragment: ${info}>`);
+                        break;
+                    }
                 }
                 node.parentNode?.insertBefore(newNode, node);
             }