diff --git a/ts/textsecure/WebAPI.ts b/ts/textsecure/WebAPI.ts
index c377c33da5aa..f7202a1cb03c 100644
--- a/ts/textsecure/WebAPI.ts
+++ b/ts/textsecure/WebAPI.ts
@@ -895,6 +895,9 @@ export function initialize({
   if (!is.string(contentProxyUrl)) {
     throw new Error('WebAPI.initialize: Invalid contentProxyUrl');
   }
+  if (proxyUrl && !is.string(proxyUrl)) {
+    throw new Error('WebAPI.initialize: Invalid proxyUrl');
+  }
   if (!is.string(version)) {
     throw new Error('WebAPI.initialize: Invalid version');
   }