diff --git a/app/pages/project/floating-ips/AttachFloatingIpModal.tsx b/app/pages/project/floating-ips/AttachFloatingIpModal.tsx
index 43ce1c0901..b8cc1fb925 100644
--- a/app/pages/project/floating-ips/AttachFloatingIpModal.tsx
+++ b/app/pages/project/floating-ips/AttachFloatingIpModal.tsx
@@ -36,12 +36,10 @@ function FloatingIpLabel({ fip }: { fip: FloatingIp }) {
 export const AttachFloatingIpModal = ({
   floatingIps,
   instance,
-  project,
   onDismiss,
 }: {
   floatingIps: Array<FloatingIp>
   instance: Instance
-  project: string
   onDismiss: () => void
 }) => {
   const queryClient = useApiQueryClient()
@@ -88,8 +86,7 @@ export const AttachFloatingIpModal = ({
         disabled={!floatingIp}
         onAction={() =>
           floatingIpAttach.mutate({
-            path: { floatingIp },
-            query: { project },
+            path: { floatingIp }, // note that this is an ID!
             body: { kind: 'instance', parent: instance.id },
           })
         }
diff --git a/app/pages/project/instances/instance/tabs/NetworkingTab.tsx b/app/pages/project/instances/instance/tabs/NetworkingTab.tsx
index 309f68b317..1315fcd6f7 100644
--- a/app/pages/project/instances/instance/tabs/NetworkingTab.tsx
+++ b/app/pages/project/instances/instance/tabs/NetworkingTab.tsx
@@ -335,7 +335,6 @@ export function NetworkingTab() {
             floatingIps={availableIps}
             instance={instance}
             onDismiss={() => setAttachModalOpen(false)}
-            project={project}
           />
         )}
       </TableControls>
diff --git a/mock-api/msw/db.ts b/mock-api/msw/db.ts
index e54ad02450..fb75391799 100644
--- a/mock-api/msw/db.ts
+++ b/mock-api/msw/db.ts
@@ -14,6 +14,7 @@ import * as mock from '@oxide/api-mocks'
 
 import { json } from '~/api/__generated__/msw-handlers'
 import { pick } from '~/util/object'
+import { commaSeries } from '~/util/str'
 
 import type { Json } from '../json-type'
 import { internalError } from './util'
@@ -25,6 +26,11 @@ export const notFoundErr = (msg?: string) => {
   return json({ error_code: 'ObjectNotFound', message } as const, { status: 404 })
 }
 
+export const badSelectorErr = (resource: string, parents: string[]) => {
+  const message = `when ${resource} is specified by ID, ${commaSeries(parents, 'and')} should not be specified`
+  return json({ error_code: 'InvalidRequest', message }, { status: 400 })
+}
+
 export const lookupById = <T extends { id: string }>(table: T[], id: string) => {
   const item = table.find((i) => i.id === id)
   if (!item) throw notFoundErr
@@ -85,7 +91,10 @@ export const lookup = {
   floatingIp({ floatingIp: id, ...projectSelector }: PP.FloatingIp): Json<Api.FloatingIp> {
     if (!id) throw notFoundErr
 
-    if (isUuid(id)) return lookupById(db.floatingIps, id)
+    if (isUuid(id)) {
+      if (projectSelector.project) throw badSelectorErr('floating IP', ['project'])
+      return lookupById(db.floatingIps, id)
+    }
 
     const project = lookup.project(projectSelector)
     const floatingIp = db.floatingIps.find(