diff --git a/src/lib-fs/fs-posix.c b/src/lib-fs/fs-posix.c
index a4baeae36d..43bd26180b 100644
--- a/src/lib-fs/fs-posix.c
+++ b/src/lib-fs/fs-posix.c
@@ -495,6 +495,7 @@ static int fs_posix_write(struct fs_file *_file, const void *data, size_t size)
 	if (file->fd == -1) {
 		if (fs_posix_open(file) < 0)
 			return -1;
+		i_assert(file->fd != -1);
 	}
 
 	if (file->open_mode != FS_OPEN_MODE_APPEND) {
diff --git a/src/lib-index/mailbox-log.c b/src/lib-index/mailbox-log.c
index 33305171b4..11f1824ef7 100644
--- a/src/lib-index/mailbox-log.c
+++ b/src/lib-index/mailbox-log.c
@@ -176,6 +176,7 @@ int mailbox_log_append(struct mailbox_log *log,
 	if (log->fd == -1) {
 		if (mailbox_log_open(log) < 0)
 			return -1;
+		i_assert(log->fd != -1);
 	}
 
 	/* We don't bother with locking, atomic appends will protect us.
diff --git a/src/lib-stats/stats-connection.c b/src/lib-stats/stats-connection.c
index f342626656..6172abc807 100644
--- a/src/lib-stats/stats-connection.c
+++ b/src/lib-stats/stats-connection.c
@@ -83,6 +83,7 @@ int stats_connection_send(struct stats_connection *conn, const string_t *str)
 	if (conn->fd == -1) {
 		if (!stats_connection_open(conn))
 			return -1;
+		i_assert(conn->fd != -1);
 	}
 
 	if (str_len(str) > PIPE_BUF && !pipe_warned) {
diff --git a/src/lib-storage/index/imapc/imapc-mail.c b/src/lib-storage/index/imapc/imapc-mail.c
index b520e4e69e..b040e2d6f5 100644
--- a/src/lib-storage/index/imapc/imapc-mail.c
+++ b/src/lib-storage/index/imapc/imapc-mail.c
@@ -187,6 +187,7 @@ static int imapc_mail_get_physical_size(struct mail *_mail, uoff_t *size_r)
 	old_offset = data->stream == NULL ? 0 : data->stream->v_offset;
 	if (mail_get_stream(_mail, NULL, NULL, &input) < 0)
 		return -1;
+	i_assert(data->stream != NULL);
 	i_stream_seek(data->stream, old_offset);
 
 	ret = i_stream_get_size(data->stream, TRUE,
diff --git a/src/lib/istream-file.c b/src/lib/istream-file.c
index 15cdc21e76..da25090c45 100644
--- a/src/lib/istream-file.c
+++ b/src/lib/istream-file.c
@@ -54,6 +54,7 @@ ssize_t i_stream_file_read(struct istream_private *stream)
 	if (stream->fd == -1) {
 		if (i_stream_file_open(stream) < 0)
 			return -1;
+		i_assert(stream->fd != -1);
 	}
 
 	offset = stream->istream.v_offset + (stream->pos - stream->skip);
diff --git a/src/login-common/login-proxy-state.c b/src/login-common/login-proxy-state.c
index 1200904ff2..4c82345df8 100644
--- a/src/login-common/login-proxy-state.c
+++ b/src/login-common/login-proxy-state.c
@@ -138,6 +138,7 @@ static bool login_proxy_state_try_notify(struct login_proxy_state *state,
 	if (state->notify_fd == -1) {
 		if (login_proxy_state_notify_open(state) < 0)
 			return TRUE;
+		i_assert(state->notify_fd != -1);
 	}
 
 	T_BEGIN {
diff --git a/src/plugins/fts/fts-api.c b/src/plugins/fts/fts-api.c
index 5eec770bc8..a12c1903d3 100644
--- a/src/plugins/fts/fts-api.c
+++ b/src/plugins/fts/fts-api.c
@@ -125,6 +125,7 @@ static void fts_backend_set_cur_mailbox(struct fts_backend_update_context *ctx)
 {
 	fts_backend_update_unset_build_key(ctx);
 	if (ctx->backend_box != ctx->cur_box) {
+		i_assert(ctx->cur_box != NULL);
 		ctx->backend->v.update_set_mailbox(ctx, ctx->cur_box);
 		ctx->backend_box = ctx->cur_box;
 	}
diff --git a/src/plugins/virtual/virtual-mail.c b/src/plugins/virtual/virtual-mail.c
index 78443266d9..21e7519091 100644
--- a/src/plugins/virtual/virtual-mail.c
+++ b/src/plugins/virtual/virtual-mail.c
@@ -131,6 +131,7 @@ static int backend_mail_get(struct virtual_mail *vmail,
 			return -1;
 		}
 		(void)virtual_mail_set_backend_mail(mail, bbox);
+		i_assert(vmail->cur_backend_mail != NULL);
 	}
 	virtual_backend_box_accessed(mbox, bbox);
 	vmail->cur_lost = !mail_set_uid(vmail->cur_backend_mail,