diff --git a/src/vmspawn/vmspawn.c b/src/vmspawn/vmspawn.c
index 8fad195abcd..030efbdb23f 100644
--- a/src/vmspawn/vmspawn.c
+++ b/src/vmspawn/vmspawn.c
@@ -626,10 +626,10 @@ static int read_vsock_notify(NotifyConnectionData *d, int fd) {
                 if (n == 0) /* We hit EOF! Let's parse this */
                         break;
 
-                d->full += n;
-
-                if (d->full >= sizeof(d->buffer))
+                if ((size_t) n >= sizeof(d->buffer) - d->full)
                         return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Received notify message exceeded maximum size.");
+
+                d->full += n;
         }
 
         /* We reached EOF, now parse the thing */