diff --git a/Libraries/LibProtocol/Client.cpp b/Libraries/LibProtocol/Client.cpp
index 2c1973291ef..0b80102a8d7 100644
--- a/Libraries/LibProtocol/Client.cpp
+++ b/Libraries/LibProtocol/Client.cpp
@@ -50,6 +50,8 @@ bool Client::is_supported_protocol(const String& protocol)
 RefPtr<Download> Client::start_download(const String& url)
 {
     i32 download_id = send_sync<Messages::ProtocolServer::StartDownload>(url)->download_id();
+    if (download_id < 0)
+        return nullptr;
     auto download = Download::create_from_id({}, *this, download_id);
     m_downloads.set(download_id, download);
     return download;
diff --git a/Libraries/LibWeb/ResourceLoader.cpp b/Libraries/LibWeb/ResourceLoader.cpp
index c6b1aac24c3..7ef40f5cf54 100644
--- a/Libraries/LibWeb/ResourceLoader.cpp
+++ b/Libraries/LibWeb/ResourceLoader.cpp
@@ -86,6 +86,11 @@ void ResourceLoader::load(const URL& url, Function<void(const ByteBuffer&)> succ
 
     if (url.protocol() == "http") {
         auto download = protocol_client().start_download(url.to_string());
+        if (!download) {
+            if (error_callback)
+                error_callback("Failed to initiate load");
+            return;
+        }
         download->on_finish = [this, success_callback = move(success_callback), error_callback = move(error_callback)](bool success, const ByteBuffer& payload, auto) {
             --m_pending_loads;
             if (on_load_counter_change)
diff --git a/Servers/ProtocolServer/PSClientConnection.cpp b/Servers/ProtocolServer/PSClientConnection.cpp
index e788e190f3f..b8be911f841 100644
--- a/Servers/ProtocolServer/PSClientConnection.cpp
+++ b/Servers/ProtocolServer/PSClientConnection.cpp
@@ -57,9 +57,11 @@ OwnPtr<Messages::ProtocolServer::IsSupportedProtocolResponse> PSClientConnection
 OwnPtr<Messages::ProtocolServer::StartDownloadResponse> PSClientConnection::handle(const Messages::ProtocolServer::StartDownload& message)
 {
     URL url(message.url());
-    ASSERT(url.is_valid());
+    if (!url.is_valid())
+        return make<Messages::ProtocolServer::StartDownloadResponse>(-1);
     auto* protocol = Protocol::find_by_name(url.protocol());
-    ASSERT(protocol);
+    if (!protocol)
+        return make<Messages::ProtocolServer::StartDownloadResponse>(-1);
     auto download = protocol->start_download(*this, url);
     return make<Messages::ProtocolServer::StartDownloadResponse>(download->id());
 }
diff --git a/Userland/pro.cpp b/Userland/pro.cpp
index 788e42427f7..51d0b4912f2 100644
--- a/Userland/pro.cpp
+++ b/Userland/pro.cpp
@@ -49,6 +49,10 @@ int main(int argc, char** argv)
     auto protocol_client = Protocol::Client::construct();
 
     auto download = protocol_client->start_download(url.to_string());
+    if (!download) {
+        fprintf(stderr, "Failed to start download for '%s'\n", url_string.characters());
+        return 1;
+    }
     download->on_progress = [](u32 total_size, u32 downloaded_size) {
         dbgprintf("download progress: %u / %u\n", downloaded_size, total_size);
     };