diff --git a/src/ckpool.c b/src/ckpool.c
index e08cbc2f..00a7d456 100644
--- a/src/ckpool.c
+++ b/src/ckpool.c
@@ -399,6 +399,71 @@ out:
 	return buf;
 }
 
+static const char *invalid_unknown = " (unknown reason)";
+static const char *invalid_toodeep = " >9 levels, recursion?";
+
+#define first_invalid(_json_data) _first_invalid(_json_data, 0)
+
+static char *_first_invalid(json_t *json_data, int level)
+{
+	const char *json_key;
+	json_t *json_value;
+	void *json_iter;
+	int json_typ;
+	char buf[512], *inside;
+	bool found;
+
+	if (level > 9)
+		return strdup(invalid_toodeep);
+
+	buf[0] = '\0';
+	found = false;
+	json_iter = json_object_iter(json_data);	
+	while (!found && json_iter) {
+		json_key = json_object_iter_key(json_iter);
+		json_value = json_object_iter_value(json_iter);
+		json_typ = json_typeof(json_value);
+		switch(json_typ) {
+			case JSON_STRING:
+			case JSON_REAL:
+			case JSON_INTEGER:
+			case JSON_TRUE:
+			case JSON_FALSE:
+				break;
+			case JSON_ARRAY:
+				inside = _first_invalid(json_value, level+1);
+				if (inside != invalid_unknown) {
+					snprintf(buf, sizeof(buf),
+						 " %s : [%s ]", json_key, inside);
+					free(inside);
+					found = true;
+				}
+				break;
+			case JSON_NULL:
+				snprintf(buf, sizeof(buf),
+					 " %s is NULL", json_key);
+				found = true;
+				break;
+			default:
+				snprintf(buf, sizeof(buf),
+					 " unknown type %d for %s",
+					 json_typ, json_key);
+				found = true;
+				break;
+		}
+		if (!found)
+			json_iter = json_object_iter_next(json_data, json_iter);
+	}
+
+	if (!*buf) {
+		if (level > 0)
+			return (char *)invalid_unknown;
+		else
+			return strdup(invalid_unknown);
+	} else
+		return strdup(buf);
+}
+
 /* Send a json msg to ckdb with its idmsg and return the response, consuming
  * the json on success */
 char *_json_ckdb_call(const ckpool_t *ckp, const char *idmsg, json_t *val, bool logged,
@@ -408,7 +473,9 @@ char *_json_ckdb_call(const ckpool_t *ckp, const char *idmsg, json_t *val, bool
 
 	dump = json_dumps(val, JSON_COMPACT);
 	if (unlikely(!dump)) {
-		LOGWARNING("Json dump failed in json_ckdb_call from %s %s:%d", file, func, line);
+		char *invalid = first_invalid(val);
+		LOGWARNING("Json dump failed in json_ckdb_call from %s %s:%d%s", file, func, line, invalid);
+		free(invalid);
 		return buf;
 	}
 	ASPRINTF(&msg, "%s.id.json=%s", idmsg, dump);