diff --git a/backend/routes/auth/discord.go b/backend/routes/auth/discord.go
index 7ef3b04..34b0f93 100644
--- a/backend/routes/auth/discord.go
+++ b/backend/routes/auth/discord.go
@@ -193,6 +193,11 @@ func (s *Server) discordLink(w http.ResponseWriter, r *http.Request) error {
 		return server.APIError{Code: server.ErrInvalidTicket}
 	}
 
+	if du.ID == "" {
+		log.Errorf("linking user with id %v: discord user ID was empty", claims.UserID)
+		return server.APIError{Code: server.ErrInternalServerError, Details: "Discord user ID is empty"}
+	}
+
 	err = u.UpdateFromDiscord(ctx, s.DB, du)
 	if err != nil {
 		return errors.Wrap(err, "updating user from discord")
@@ -302,6 +307,11 @@ func (s *Server) discordSignup(w http.ResponseWriter, r *http.Request) error {
 		return errors.Wrap(err, "creating user")
 	}
 
+	if du.ID == "" {
+		log.Errorf("creating user with name %q: user ID was empty", req.Username)
+		return server.APIError{Code: server.ErrInternalServerError, Details: "Discord user ID is empty"}
+	}
+
 	err = u.UpdateFromDiscord(ctx, tx, du)
 	if err != nil {
 		return errors.Wrap(err, "updating user from discord")
diff --git a/backend/routes/auth/fedi_mastodon.go b/backend/routes/auth/fedi_mastodon.go
index aece379..85bab13 100644
--- a/backend/routes/auth/fedi_mastodon.go
+++ b/backend/routes/auth/fedi_mastodon.go
@@ -220,6 +220,11 @@ func (s *Server) mastodonLink(w http.ResponseWriter, r *http.Request) error {
 		return server.APIError{Code: server.ErrInvalidTicket}
 	}
 
+	if mu.ID == "" {
+		log.Errorf("linking user with id %v: user ID was empty", claims.UserID)
+		return server.APIError{Code: server.ErrInternalServerError, Details: "Mastodon user ID is empty"}
+	}
+
 	err = u.UpdateFromFedi(ctx, s.DB, mu.ID, mu.Username, app.ID)
 	if err != nil {
 		return errors.Wrap(err, "updating user from mastoAPI")
@@ -330,6 +335,11 @@ func (s *Server) mastodonSignup(w http.ResponseWriter, r *http.Request) error {
 		return errors.Wrap(err, "creating user")
 	}
 
+	if mu.ID == "" {
+		log.Errorf("creating user with name %q: user ID was empty", req.Username)
+		return server.APIError{Code: server.ErrInternalServerError, Details: "Mastodon user ID is empty"}
+	}
+
 	err = u.UpdateFromFedi(ctx, tx, mu.ID, mu.Username, app.ID)
 	if err != nil {
 		return errors.Wrap(err, "updating user from mastoAPI")
diff --git a/backend/routes/auth/fedi_misskey.go b/backend/routes/auth/fedi_misskey.go
index 9d869ad..69b0d94 100644
--- a/backend/routes/auth/fedi_misskey.go
+++ b/backend/routes/auth/fedi_misskey.go
@@ -195,6 +195,11 @@ func (s *Server) misskeyLink(w http.ResponseWriter, r *http.Request) error {
 		return server.APIError{Code: server.ErrInvalidTicket}
 	}
 
+	if mu.ID == "" {
+		log.Errorf("linking user with id %v: user ID was empty", claims.UserID)
+		return server.APIError{Code: server.ErrInternalServerError, Details: "Misskey user ID is empty"}
+	}
+
 	err = u.UpdateFromFedi(ctx, s.DB, mu.ID, mu.Username, app.ID)
 	if err != nil {
 		return errors.Wrap(err, "updating user from misskey")
@@ -260,6 +265,11 @@ func (s *Server) misskeySignup(w http.ResponseWriter, r *http.Request) error {
 		return errors.Wrap(err, "creating user")
 	}
 
+	if mu.ID == "" {
+		log.Errorf("creating user with name %q: user ID was empty", req.Username)
+		return server.APIError{Code: server.ErrInternalServerError, Details: "Misskey user ID is empty"}
+	}
+
 	err = u.UpdateFromFedi(ctx, tx, mu.ID, mu.Username, app.ID)
 	if err != nil {
 		return errors.Wrap(err, "updating user from misskey")
diff --git a/backend/routes/auth/google.go b/backend/routes/auth/google.go
index d28b48b..9edb21e 100644
--- a/backend/routes/auth/google.go
+++ b/backend/routes/auth/google.go
@@ -208,6 +208,11 @@ func (s *Server) googleLink(w http.ResponseWriter, r *http.Request) error {
 		return server.APIError{Code: server.ErrInvalidTicket}
 	}
 
+	if gu.ID == "" {
+		log.Errorf("linking user with id %v: user ID was empty", claims.UserID)
+		return server.APIError{Code: server.ErrInternalServerError, Details: "Google user ID is empty"}
+	}
+
 	err = u.UpdateFromGoogle(ctx, s.DB, gu.ID, gu.Email)
 	if err != nil {
 		return errors.Wrap(err, "updating user from google")
@@ -306,6 +311,11 @@ func (s *Server) googleSignup(w http.ResponseWriter, r *http.Request) error {
 		return errors.Wrap(err, "creating user")
 	}
 
+	if gu.ID == "" {
+		log.Errorf("creating user with name %q: user ID was empty", req.Username)
+		return server.APIError{Code: server.ErrInternalServerError, Details: "Google user ID is empty"}
+	}
+
 	err = u.UpdateFromGoogle(ctx, tx, gu.ID, gu.Email)
 	if err != nil {
 		return errors.Wrap(err, "updating user from google")
diff --git a/backend/routes/auth/tumblr.go b/backend/routes/auth/tumblr.go
index 22ee4e4..8fdddf1 100644
--- a/backend/routes/auth/tumblr.go
+++ b/backend/routes/auth/tumblr.go
@@ -241,6 +241,11 @@ func (s *Server) tumblrLink(w http.ResponseWriter, r *http.Request) error {
 		return server.APIError{Code: server.ErrInvalidTicket}
 	}
 
+	if tui.ID == "" {
+		log.Errorf("linking user with id %v: user ID was empty", claims.UserID)
+		return server.APIError{Code: server.ErrInternalServerError, Details: "Tumblr user ID is empty"}
+	}
+
 	err = u.UpdateFromTumblr(ctx, s.DB, tui.ID, tui.Name)
 	if err != nil {
 		return errors.Wrap(err, "updating user from tumblr")
@@ -339,6 +344,11 @@ func (s *Server) tumblrSignup(w http.ResponseWriter, r *http.Request) error {
 		return errors.Wrap(err, "creating user")
 	}
 
+	if tui.ID == "" {
+		log.Errorf("creating user with name %q: user ID was empty", req.Username)
+		return server.APIError{Code: server.ErrInternalServerError, Details: "Tumblr user ID is empty"}
+	}
+
 	err = u.UpdateFromTumblr(ctx, tx, tui.ID, tui.Name)
 	if err != nil {
 		return errors.Wrap(err, "updating user from tumblr")