diff --git a/src/app.rs b/src/app.rs
index 070fabd..a517be8 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -495,10 +495,22 @@ fn CallPanel(room: Memo<String>, me: String) -> impl IntoView {
         }
             .into_any()
     }
+    // Must mirror the hydrate branch's default (not-in-call) markup exactly -
+    // hydration reconciles this SSR output against what the hydrate branch
+    // above expects to find, and an empty div here (vs. the button hydrate
+    // wants) is a hydration mismatch that panics and traps the whole wasm
+    // instance, killing all reactivity on the page.
     #[cfg(not(feature = "hydrate"))]
     {
         let _ = (room, me);
-        view! { <div class="call-panel"></div> }.into_any()
+        view! {
+            <div class="call-panel">
+                <button class="call-join" disabled=true>
+                    "☎ join call"
+                </button>
+            </div>
+        }
+            .into_any()
     }
 }