diff --git a/src/calanonsync/ews.go b/src/calanonsync/ews.go
index d2f8820..e44ce43 100644
--- a/src/calanonsync/ews.go
+++ b/src/calanonsync/ews.go
@@ -70,40 +70,38 @@ func (er EWSRoundTripper) RoundTrip(r *http.Request) (*http.Response, error) {
 		r.SetBasicAuth(er.username, er.password)
 	}
 
+	er.initMutex.Lock()
 	resp, err := er.delegate.RoundTrip(r)
-	if err == nil && resp.StatusCode == http.StatusUnauthorized {
-		er.initMutex.Lock()
-		defer er.initMutex.Unlock()
-
-		if er.authType == authTypeUnknown {
-			// This is a good time to find out what the server prefers.
-			authHeaders := resp.Header["Www-Authenticate"]
-			if authHeaders != nil {
-				for _, h := range authHeaders {
-					if strings.HasPrefix(h, "BASIC") {
-						er.authType = authTypeBasic
-					} else if strings.HasPrefix(h, "NTLM") {
-						er.authType = authTypeNTLM
-						break // NTLM is the best we could do
-					}
+	if err == nil && resp.StatusCode == http.StatusUnauthorized && er.authType == authTypeUnknown {
+		// This is a good time to find out what the server prefers.
+		authHeaders := resp.Header["Www-Authenticate"]
+		if authHeaders != nil {
+			for _, h := range authHeaders {
+				if strings.HasPrefix(h, "BASIC") {
+					er.authType = authTypeBasic
+				} else if strings.HasPrefix(h, "NTLM") {
+					er.authType = authTypeNTLM
+					break // NTLM is the best we could do
 				}
 			}
+		}
 
-			// So, do we know more than before? If so, try again.
-			if er.authType > authTypeUnknown {
-				if er.authType == authTypeNTLM {
-					// We need to replace the delegator.
-					er.delegate = &httpntlm.NtlmTransport{
-						Domain:   "",
-						User:     er.username,
-						Password: er.password,
-					}
+		// So, do we know more than before? If so, try again.
+		if er.authType > authTypeUnknown {
+			if er.authType == authTypeNTLM {
+				// We need to replace the delegator.
+				er.delegate = &httpntlm.NtlmTransport{
+					Domain:   "",
+					User:     er.username,
+					Password: er.password,
 				}
-				return er.RoundTrip(r)
 			}
+			er.initMutex.Unlock()
+			return er.RoundTrip(r)
 		}
 	}
 
+	er.initMutex.Unlock()
 	return resp, err
 }