diff --git a/relay-java/src/main/java/com/genymobile/gnirehtet/relay/DatagramBuffer.java b/relay-java/src/main/java/com/genymobile/gnirehtet/relay/DatagramBuffer.java
index 9516bcb..9608bd9 100644
--- a/relay-java/src/main/java/com/genymobile/gnirehtet/relay/DatagramBuffer.java
+++ b/relay-java/src/main/java/com/genymobile/gnirehtet/relay/DatagramBuffer.java
@@ -65,8 +65,8 @@ public boolean hasEnoughSpaceFor(int datagramLength) {
             // there is at least the extra space for storing 1 packet
             return true;
         }
-        int remaining = tail - head - 1;
-        return HEADER_LENGTH + datagramLength < remaining;
+        int remaining = tail - head - 1; // 1 extra byte to distinguish empty vs full
+        return HEADER_LENGTH + datagramLength <= remaining;
     }
 
     public int capacity() {
diff --git a/relay-rust/src/relay/datagram_buffer.rs b/relay-rust/src/relay/datagram_buffer.rs
index 91bfcde..7ab6084 100644
--- a/relay-rust/src/relay/datagram_buffer.rs
+++ b/relay-rust/src/relay/datagram_buffer.rs
@@ -63,8 +63,9 @@ impl DatagramBuffer {
             // there is at least the extra space for storing 1 packet
             return true;
         }
-        let remaining = self.tail - self.head + 1;
-        HEADER_LENGTH + datagram_length < remaining
+        // 1 extra byte to distinguish empty vs full
+        let remaining = self.tail - self.head - 1;
+        HEADER_LENGTH + datagram_length <= remaining
     }
 
     pub fn write_to<S: DatagramSender>(&mut self, destination: &mut S) -> io::Result<()> {