diff --git a/internal/model/load.go b/internal/model/load.go
index 97d3a7f..4f46e49 100644
--- a/internal/model/load.go
+++ b/internal/model/load.go
@@ -16,6 +16,9 @@ func LoadCluster(path string) (Cluster, error) {
 	if err := loadYAML(path, "cluster", &cluster); err != nil {
 		return Cluster{}, err
 	}
+	if cluster.Nodes == nil {
+		return Cluster{}, fmt.Errorf("validate cluster file %q: top-level field %q is required and must be a sequence", path, "nodes")
+	}
 	if err := ValidateCluster(cluster); err != nil {
 		return Cluster{}, fmt.Errorf("validate cluster file %q: %w", path, err)
 	}
@@ -28,6 +31,9 @@ func LoadJobs(path string) (JobSet, error) {
 	if err := loadYAML(path, "jobs", &jobs); err != nil {
 		return JobSet{}, err
 	}
+	if jobs.Jobs == nil {
+		return JobSet{}, fmt.Errorf("validate jobs file %q: top-level field %q is required and must be a sequence", path, "jobs")
+	}
 	if err := ValidateJobs(jobs); err != nil {
 		return JobSet{}, fmt.Errorf("validate jobs file %q: %w", path, err)
 	}
diff --git a/internal/model/load_test.go b/internal/model/load_test.go
index 662c57f..26e84ee 100644
--- a/internal/model/load_test.go
+++ b/internal/model/load_test.go
@@ -34,6 +34,26 @@ func TestLoadCluster(t *testing.T) {
 				Capacity: model.Resources{CPU: 32, GPU: 4},
 			}}},
 		},
+		{
+			name:     "explicit empty sequence",
+			contents: "nodes: []\n",
+			want:     model.Cluster{Nodes: []model.Node{}},
+		},
+		{
+			name:        "missing nodes field",
+			contents:    "{}\n",
+			wantErrText: `top-level field "nodes" is required and must be a sequence`,
+		},
+		{
+			name:        "null document",
+			contents:    "null\n",
+			wantErrText: `top-level field "nodes" is required and must be a sequence`,
+		},
+		{
+			name:        "null nodes field",
+			contents:    "nodes: null\n",
+			wantErrText: `top-level field "nodes" is required and must be a sequence`,
+		},
 		{
 			name: "unknown root field",
 			contents: `nodes: []
@@ -135,6 +155,26 @@ func TestLoadJobs(t *testing.T) {
 				{Name: "second", Replicas: 1, ResourcesPerReplica: model.Resources{GPU: 1}, RequiredTopology: "rack"},
 			}},
 		},
+		{
+			name:     "explicit empty sequence",
+			contents: "jobs: []\n",
+			want:     model.JobSet{Jobs: []model.Job{}},
+		},
+		{
+			name:        "missing jobs field",
+			contents:    "{}\n",
+			wantErrText: `top-level field "jobs" is required and must be a sequence`,
+		},
+		{
+			name:        "null document",
+			contents:    "null\n",
+			wantErrText: `top-level field "jobs" is required and must be a sequence`,
+		},
+		{
+			name:        "null jobs field",
+			contents:    "jobs: null\n",
+			wantErrText: `top-level field "jobs" is required and must be a sequence`,
+		},
 		{
 			name: "unknown job field",
 			contents: `jobs: