diff --git a/src/mon/Monitor.cc b/src/mon/Monitor.cc
index 10c8bfc..98843d7 100644
--- a/src/mon/Monitor.cc
+++ b/src/mon/Monitor.cc
@@ -2631,7 +2631,19 @@ void Monitor::handle_command(MonOpRequestRef op)
     return;
   }
 
-  cmd_getval(g_ceph_context, cmdmap, "prefix", prefix);
+  // check return value. If no prefix parameter provided,
+  // return value will be false, then return error info.
+  if(!cmd_getval(g_ceph_context, cmdmap, "prefix", prefix)) {
+    reply_command(op, -EINVAL, "command prefix not found", 0);
+    return;
+  }
+
+  // check prefix is empty
+  if (prefix.empty()) {
+    reply_command(op, -EINVAL, "command prefix must not be empty", 0);
+    return;
+  }
+
   if (prefix == "get_command_descriptions") {
     bufferlist rdata;
     Formatter *f = Formatter::create("json");
@@ -2652,6 +2664,15 @@ void Monitor::handle_command(MonOpRequestRef op)
   boost::scoped_ptr<Formatter> f(Formatter::create(format));
 
   get_str_vec(prefix, fullcmd);
+
+  // make sure fullcmd is not empty.
+  // invalid prefix will cause empty vector fullcmd.
+  // such as, prefix=";,,;"
+  if (fullcmd.empty()) {
+    reply_command(op, -EINVAL, "command requires a prefix to be valid", 0);
+    return;
+  }
+
   module = fullcmd[0];
 
   // validate command is in leader map
diff --git a/src/test/librados/cmd.cc b/src/test/librados/cmd.cc
index 9261fb5..878a8af 100644
--- a/src/test/librados/cmd.cc
+++ b/src/test/librados/cmd.cc
@@ -48,6 +48,41 @@ TEST(LibRadosCmd, MonDescribe) {
   rados_buffer_free(buf);
   rados_buffer_free(st);
 
+  cmd[0] = (char *)"";
+  ASSERT_EQ(-EINVAL, rados_mon_command(cluster, (const char **)cmd, 1, "{}", 2, &buf, &buflen, &st, &stlen));
+  rados_buffer_free(buf);
+  rados_buffer_free(st);
+
+  cmd[0] = (char *)"{}";
+  ASSERT_EQ(-EINVAL, rados_mon_command(cluster, (const char **)cmd, 1, "", 0, &buf, &buflen, &st, &stlen));
+  rados_buffer_free(buf);
+  rados_buffer_free(st);
+
+  cmd[0] = (char *)"{\"abc\":\"something\"}";
+  ASSERT_EQ(-EINVAL, rados_mon_command(cluster, (const char **)cmd, 1, "", 0, &buf, &buflen, &st, &stlen));
+  rados_buffer_free(buf);
+  rados_buffer_free(st);
+
+  cmd[0] = (char *)"{\"prefix\":\"\"}";
+  ASSERT_EQ(-EINVAL, rados_mon_command(cluster, (const char **)cmd, 1, "", 0, &buf, &buflen, &st, &stlen));
+  rados_buffer_free(buf);
+  rados_buffer_free(st);
+
+  cmd[0] = (char *)"{\"prefix\":\"    \"}";
+  ASSERT_EQ(-EINVAL, rados_mon_command(cluster, (const char **)cmd, 1, "", 0, &buf, &buflen, &st, &stlen));
+  rados_buffer_free(buf);
+  rados_buffer_free(st);
+
+  cmd[0] = (char *)"{\"prefix\":\";;;,,,;;,,\"}";
+  ASSERT_EQ(-EINVAL, rados_mon_command(cluster, (const char **)cmd, 1, "", 0, &buf, &buflen, &st, &stlen));
+  rados_buffer_free(buf);
+  rados_buffer_free(st);
+
+  cmd[0] = (char *)"{\"prefix\":\"extra command\"}";
+  ASSERT_EQ(-EINVAL, rados_mon_command(cluster, (const char **)cmd, 1, "", 0, &buf, &buflen, &st, &stlen));
+  rados_buffer_free(buf);
+  rados_buffer_free(st);
+
   cmd[0] = (char *)"{\"prefix\":\"mon_status\"}";
   ASSERT_EQ(0, rados_mon_command(cluster, (const char **)cmd, 1, "", 0, &buf, &buflen, &st, &stlen));
   ASSERT_LT(0u, buflen);