diff --git a/include/libwebsockets/lws-dll2.h b/include/libwebsockets/lws-dll2.h
index a4542aa25..478aa4344 100644
--- a/include/libwebsockets/lws-dll2.h
+++ b/include/libwebsockets/lws-dll2.h
@@ -207,8 +207,8 @@ typedef struct lws_dll2_owner {
 	uint32_t		count;
 } lws_dll2_owner_t;
 
-static LWS_INLINE int
-lws_dll2_is_detached(const struct lws_dll2 *d) { return !d->owner; }
+LWS_VISIBLE LWS_EXTERN int
+lws_dll2_is_detached(const struct lws_dll2 *d);
 
 static LWS_INLINE const struct lws_dll2_owner *
 lws_dll2_owner(const struct lws_dll2 *d) { return d->owner; }
diff --git a/lib/core/lws_dll2.c b/lib/core/lws_dll2.c
index 9b9e6b37e..18888dd07 100644
--- a/lib/core/lws_dll2.c
+++ b/lib/core/lws_dll2.c
@@ -28,6 +28,31 @@
 #include <sys/types.h>
 #endif
 
+int
+lws_dll2_is_detached(const struct lws_dll2 *d)
+{
+	if (d->owner)
+		return 0;
+
+	if (d->next || d->prev) {
+		lwsl_err("%s: dll2 %p: detached but next %p, prev %p\n",
+				__func__, d, d->next, d->prev);
+		/*
+		 * New lws_dll2 objects and removed lws_dll2 objects
+		 * have .owner, .next and .prev all set to NULL, so we
+		 * can just check .owner to see if we are detached.
+		 *
+		 * We assert here if we encounter an lws_dll2 in the illegal
+		 * state of NULL .owner, but non-NULL in .next or .prev,
+		 * it's evidence of corruption, use-after-free, threads
+		 * contending on accessing without locking etc.
+		 */
+		assert(0);
+	}
+
+	return 1;
+}
+
 int
 lws_dll2_foreach_safe(struct lws_dll2_owner *owner, void *user,
 		      int (*cb)(struct lws_dll2 *d, void *user))