diff --git a/internal/measurex/archival.go b/internal/measurex/archival.go
index 2feb65f..055d250 100644
--- a/internal/measurex/archival.go
+++ b/internal/measurex/archival.go
@@ -496,11 +496,14 @@ type ArchivalTHMeasurement struct {
 }
 
 // NewArchivalTHMeasurement creates the archival representation of THMeasurement.
-func NewArchivalTHMeasurement(in *THMeasurement) *ArchivalTHMeasurement {
-	return &ArchivalTHMeasurement{
-		DNS:       NewArchivalDNSMeasurementList(in.DNS),
-		Endpoints: NewArchivalHTTPEndpointMeasurementList(in.Endpoints),
+func NewArchivalTHMeasurement(in *THMeasurement) (out *ArchivalTHMeasurement) {
+	if in != nil {
+		out = &ArchivalTHMeasurement{
+			DNS:       NewArchivalDNSMeasurementList(in.DNS),
+			Endpoints: NewArchivalHTTPEndpointMeasurementList(in.Endpoints),
+		}
 	}
+	return
 }
 
 //
diff --git a/internal/measurex/measurement.go b/internal/measurex/measurement.go
index 1d04652..0cac00a 100644
--- a/internal/measurex/measurement.go
+++ b/internal/measurex/measurement.go
@@ -31,7 +31,8 @@ type URLMeasurement struct {
 	// if we choose to follow redirections.
 	RedirectURLs []string
 
-	// TH is the measurement collected by the TH.
+	// TH is the measurement collected by the TH. This field
+	// will be nil if we cannot contact the TH.
 	TH *THMeasurement
 
 	// TotalRuntime is the total time to measure this URL.