diff --git a/changes/bug18859 b/changes/bug18859
new file mode 100644
index 0000000000..993074079c
--- /dev/null
+++ b/changes/bug18859
@@ -0,0 +1,5 @@
+  o Minor bugfixes (circuit prediction):
+    - Fix circuit_stream_is_being_handled so it correctly reports on circuits
+      with isolation settings. Ports must not be said to be "handled" by
+      already-isolated circuits, and a stream can only be handled by a circuit
+      if their isolation settings are compatible. Fixes bug 18859.
diff --git a/src/or/circuituse.c b/src/or/circuituse.c
index 9f9d3abf7c..6378cb8ac3 100644
--- a/src/or/circuituse.c
+++ b/src/or/circuituse.c
@@ -973,7 +973,7 @@ circuit_remove_handled_ports(smartlist_t *needed_ports)
     tor_assert(*port);
     if (circuit_stream_is_being_handled(NULL, *port,
                                         MIN_CIRCUITS_HANDLING_STREAM)) {
-//      log_debug(LD_CIRC,"Port %d is already being handled; removing.", port);
+      log_debug(LD_CIRC,"Port %d is already being handled; removing.", *port);
       smartlist_del(needed_ports, i--);
       tor_free(port);
     } else {
@@ -1010,6 +1010,10 @@ circuit_stream_is_being_handled(entry_connection_t *conn,
         continue;
       if (origin_circ->unusable_for_new_conns)
         continue;
+      if (origin_circ->isolation_values_set &&
+          (conn == NULL ||
+           !connection_edge_compatible_with_circuit(conn, origin_circ)))
+        continue;
 
       exitnode = build_state_get_exit_node(build_state);
       if (exitnode && (!need_uptime || build_state->need_uptime)) {