diff --git a/src/main/java/de/neemann/digital/core/io/telnet/ByteBuffer.java b/src/main/java/de/neemann/digital/core/io/telnet/ByteBuffer.java
index b43ddd338..2aee42ec9 100644
--- a/src/main/java/de/neemann/digital/core/io/telnet/ByteBuffer.java
+++ b/src/main/java/de/neemann/digital/core/io/telnet/ByteBuffer.java
@@ -52,8 +52,10 @@ public class ByteBuffer {
      * deletes a byte from the tail of the buffer
      */
     synchronized public void delete() {
-        oldest = inc(oldest);
-        inBuffer--;
+        if (inBuffer > 0) {
+            oldest = inc(oldest);
+            inBuffer--;
+        }
     }
 
     /**