guard against non-errors

packages/svelte/src/internal/client/dom/blocks/boundary.js

@@ -114,7 +114,7 @@ export function boundary(node, boundary_fn, props) {
 								);
 							});
 						} catch (error) {
-							handle_error(/** @type {Error} */ (error), boundary, boundary.ctx);
+							handle_error(error, boundary, boundary.ctx);
 						}
 						is_creating_fallback = false;
 					});

packages/svelte/src/internal/client/runtime.js

@@ -269,11 +269,15 @@ function should_rethrow_error(effect) {
 }
 
 /**
- * @param {Error} error
+ * @param {unknown} error
  * @param {Effect} effect
  * @param {ComponentContext | null} component_context
  */
 export function handle_error(error, effect, component_context) {
+	if (!(error instanceof Error)) {
+		throw error;
+	}
+
 	if (handled_errors.has(error)) {
 		if (should_rethrow_error(effect)) {
 			throw error;
@@ -500,7 +504,7 @@ export function update_effect(effect) {
 			dev_effect_stack.push(effect);
 		}
 	} catch (error) {
-		handle_error(/** @type {Error} */ (error), effect, previous_component_context || effect.ctx);
+		handle_error(error, effect, previous_component_context || effect.ctx);
 	} finally {
 		active_effect = previous_effect;
 
@@ -601,7 +605,7 @@ function flush_queued_effects(effects) {
 					}
 				}
 			} catch (error) {
-				handle_error(/** @type {Error} */ (error), effect, effect.ctx);
+				handle_error(error, effect, effect.ctx);
 			}
 		}
 	}
@@ -681,7 +685,7 @@ function process_effects(effect, collected_effects) {
 							update_effect(current_effect);
 						}
 					} catch (error) {
-						handle_error(/** @type {Error} */ (error), current_effect, current_effect.ctx);
+						handle_error(error, current_effect, current_effect.ctx);
 					}
 				}