diff --git a/.changeset/shiny-eels-brush.md b/.changeset/shiny-eels-brush.md
new file mode 100644
index 000000000000..d00400c31af5
--- /dev/null
+++ b/.changeset/shiny-eels-brush.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Show more descriptive error if data returned from `load` is a non-POJO
diff --git a/packages/kit/src/runtime/server/page/render.js b/packages/kit/src/runtime/server/page/render.js
index c0650e00f455..b24a9c14f54c 100644
--- a/packages/kit/src/runtime/server/page/render.js
+++ b/packages/kit/src/runtime/server/page/render.js
@@ -182,6 +182,20 @@ export async function render_response({
 				`Data returned from \`load\` while rendering ${event.routeId} is not serializable: ${error.message} (data.${match[2]})`
 			);
 		}
+
+		const nonPojoError = /pojo/i.exec(error.message);
+
+		if (nonPojoError) {
+			const constructorName = branch.find(({ server_data }) => server_data?.data?.constructor?.name)
+				?.server_data?.data?.constructor?.name;
+
+			throw new Error(
+				`Data returned from \`load\` (while rendering ${event.routeId}) must be a plain object${
+					constructorName ? ` rather than an instance of ${constructorName}` : ''
+				}`
+			);
+		}
+
 		throw error;
 	}