diff --git a/SophosSession.cs b/SophosSession.cs
index 28fac54..1b7b699 100644
--- a/SophosSession.cs
+++ b/SophosSession.cs
@@ -57,7 +57,7 @@ public async Task Login() {
             HttpResponseMessage msg = 
                     await _client.PostAsync(url, new FormUrlEncodedContent(body));
 
-            if (msg.CheckSuccess()) {
+            if (!msg.CheckSuccess()) {
                 throw new WebException("Request failed: " + msg.StatusCode);
             }