diff --git a/test/infra/control/env-isolated.bash b/test/infra/control/env-isolated.bash
index 33d2b241d..4585a2258 100755
--- a/test/infra/control/env-isolated.bash
+++ b/test/infra/control/env-isolated.bash
@@ -59,18 +59,22 @@ export TAP_DEPS="${WORKSPACE}/test/tap/tap"
 export TEST_DEPS_PATH="${WORKSPACE}/test-scripts/deps"
 export TEST_DEPS="${TEST_DEPS_PATH}"
 
-# Cluster Nodes — all run inside the ProxySQL container on different ports
-# Port scheme: proxy-node1=6042, proxy-node2=6052, ..., proxy-node9=6122
+# Cluster Nodes — primary (6032) + nodes inside the ProxySQL container
+# Port scheme: primary=6032, proxy-node1=6042, proxy-node2=6052, ..., proxy-node9=6122
 # From the test-runner container, reach them via the proxysql hostname
+# NOTE: primary MUST be first — test_cluster1-t expects conns[0] to be the primary
 NUM_CLUSTER_NODES=${PROXYSQL_CLUSTER_NODES:-9}
 if [[ "${SKIP_CLUSTER_START}" == "1" ]] || [[ "${SKIP_CLUSTER_START}" == "true" ]]; then
     NUM_CLUSTER_NODES=0
 fi
 CLUSTER_NODES=""
-for i in $(seq 1 ${NUM_CLUSTER_NODES}); do
-    PORT=$((6032 + i * 10))
-    CLUSTER_NODES="${CLUSTER_NODES}proxysql:${PORT},"
-done
+if [ "${NUM_CLUSTER_NODES}" -gt 0 ]; then
+    CLUSTER_NODES="proxysql:6032,"
+    for i in $(seq 1 ${NUM_CLUSTER_NODES}); do
+        PORT=$((6032 + i * 10))
+        CLUSTER_NODES="${CLUSTER_NODES}proxysql:${PORT},"
+    done
+fi
 export TAP_CLUSTER_NODES=${CLUSTER_NODES%,}
 
 # Build and runtime essentials