diff --git a/src/qemu/qemu_monitor_json.c b/src/qemu/qemu_monitor_json.c
index 18d1c9f51d731218c87f10b41885fbb4992cb957..cd8f1e599cc921a7eb03a03224c5df9c82c1bfd1 100644
--- a/src/qemu/qemu_monitor_json.c
+++ b/src/qemu/qemu_monitor_json.c
@@ -1364,6 +1364,9 @@ int qemuMonitorJSONGetBlockInfo(qemuMonitorPtr mon,
     virJSONValuePtr reply = NULL;
     virJSONValuePtr devices;
 
+    if (!cmd)
+        return -1;
+
     ret = qemuMonitorJSONCommand(mon, cmd, &reply);
     if (ret == 0)
         ret = qemuMonitorJSONCheckError(cmd, reply);