diff --git a/changes/bug4091_debugging b/changes/bug4091_debugging
new file mode 100644
index 0000000000..f785380318
--- /dev/null
+++ b/changes/bug4091_debugging
@@ -0,0 +1,5 @@
+  o Minor features:
+    - Add more information to a log statement that might help track down
+      bug 4091. If you're seeing "Bug: tor_addr_is_internal() called with a
+      non-IP address" messages (or any Bug messages, for that matter!),
+      please let us know about it.
diff --git a/src/common/address.c b/src/common/address.c
index 17cda42cb7..676c485897 100644
--- a/src/common/address.c
+++ b/src/common/address.c
@@ -355,7 +355,9 @@ tor_addr_is_internal(const tor_addr_t *addr, int for_listening)
 
   /* unknown address family... assume it's not safe for external use */
   /* rather than tor_assert(0) */
-  log_warn(LD_BUG, "tor_addr_is_internal() called with a non-IP address.");
+  log_warn(LD_BUG, "tor_addr_is_internal() called with a non-IP address of "
+           "type %d", (int)v_family);
+  tor_fragile_assert();
   return 1;
 }