diff --git a/cloud/apps/relay/src/idle-regional-rehome-selection.ts b/cloud/apps/relay/src/idle-regional-rehome-selection.ts
index bed20527386..f9757016416 100644
--- a/cloud/apps/relay/src/idle-regional-rehome-selection.ts
+++ b/cloud/apps/relay/src/idle-regional-rehome-selection.ts
@@ -300,7 +300,7 @@ function inlineRows(columns: ReadonlyArray<readonly [string, string]>, rows: num
 }
 
 function hostKey(row: SqlRow): string {
-  return `${String(row.user_id)} ${String(row.relay_host_id)}`
+  return `${String(row.user_id)}\u0000${String(row.relay_host_id)}`
 }
 
 function idleRehomeCandidate(
diff --git a/src/main/runtime/browser-client-download-transfers.ts b/src/main/runtime/browser-client-download-transfers.ts
index fce33c3638a..8341bacfe3f 100644
--- a/src/main/runtime/browser-client-download-transfers.ts
+++ b/src/main/runtime/browser-client-download-transfers.ts
@@ -330,5 +330,5 @@ function isDestinationExistsError(error: unknown): boolean {
 }
 
 function sessionKey(browserPageId: string, transferId: string): string {
-  return `${browserPageId} ${transferId}`
+  return `${browserPageId}\u0000${transferId}`
 }
diff --git a/src/wsl-guest/dispatcher.test.ts b/src/wsl-guest/dispatcher.test.ts
index f8ccc11c153..7312ca40214 100644
--- a/src/wsl-guest/dispatcher.test.ts
+++ b/src/wsl-guest/dispatcher.test.ts
@@ -802,7 +802,7 @@ describe('RelayDispatcher', () => {
     }
 
     // Multi-byte UTF-8, astral pairs, lone surrogates, controls, quotes, and backslashes.
-    const alphabet = 'aZ9 "\\\n\r\t éß€中𝄞😀𐀀�'
+    const alphabet = 'aZ9 "\\\n\r\t\u0000éß€中𝄞😀𐀀�'
 
     it('matches the pre-optimization sizing loop across randomized inputs', () => {
       const random = mulberry32(0xc0ffee)
