handle error scenario of empty local DC

agent/consul/config_endpoint.go

@@ -598,6 +598,10 @@ func gateWriteToSecondary(targetDC, localDC, primaryDC, kind string) error {
 	if kind != structs.PartitionExports {
 		return nil
 	}
+	if localDC == "" {
+		// This should not happen because the datacenter is defaulted in DefaultConfig.
+		return fmt.Errorf("unknown local datacenter")
+	}
 
 	if primaryDC == "" {
 		primaryDC = localDC

agent/consul/config_endpoint_test.go

@@ -2151,6 +2151,14 @@ func Test_gateWriteToSecondary(t *testing.T) {
 			},
 			wantErr: "must target the primary datacenter explicitly",
 		},
+		{
+			name: "empty local DC",
+			args: args{
+				localDC: "",
+				kind:    structs.PartitionExports,
+			},
+			wantErr: "unknown local datacenter",
+		},
 	}
 	for _, tc := range tt {
 		t.Run(tc.name, func(t *testing.T) {