Data node unable to find master node

[Miao]

I have two Windows VMs hosted on Azure. Both have ElasticSearch 7.x installed. One of them is designated as the master node, and the other a data node.

Here are the contents of the elasticsearch.yml file on the master node:

# ---------------------------------- Cluster -----------------------------------

cluster.name: xr

# ------------------------------------ Node ------------------------------------

node.name: xr-master-node
node.master: true
node.data: true 

# ---------------------------------- Network -----------------------------------

network.host: [_local_, _site_]

# --------------------------------- Discovery ----------------------------------

cluster.initial_master_nodes:  xr-master-node

And on the data node:

# ---------------------------------- Cluster -----------------------------------

cluster.name: xr

# ------------------------------------ Node ------------------------------------

node.name: xr-data-node-1
node.master: false
node.data: true

# ---------------------------------- Network -----------------------------------

network.host: [_local_, _site_]

# --------------------------------- Discovery ----------------------------------

discovery.seed_hosts: "10.0.1.4" # This is the private IP address of the master node
cluster.initial_master_nodes: xr-master-node

However, every time I try to start ElasticSearch on the data node, I see this error message:

[2019-09-19T10:49:07,567][WARN ][o.e.c.c.ClusterFormationFailureHelper] [xr-data-node-1] master not discovered yet: have discovered [{xr-data-node-1}{B3YtyECXTAC1vw1rfzYGRw}{mfdrFCMNRP-SE2d6XRAN9g}{10.0.1.5}{10.0.1.5:9300}{di}{ml.machine_memory=3757625344, xpack.installed=true, ml.max_open_jobs=20}]; discovery will continue using [10.0.1.4:9300] from hosts providers and [] from last-known cluster state; node term 0, last-accepted version 0 in term 0

What am I doing wrong?