diff --git a/tools/sky/serialdump.c b/tools/sky/serialdump.c
index deab56df4..7cc0d0e1e 100644
--- a/tools/sky/serialdump.c
+++ b/tools/sky/serialdump.c
@@ -281,6 +281,11 @@ main(int argc, char **argv)
         perror("could not read");
         exit(-1);
       }
+      if(n == 0) {
+        errno = EBADF;
+        perror("serial device disconnected");
+        exit(-1);
+      }
 
       for(i = 0; i < n; i++) {
         switch(mode) {