diff --git a/internal/api/handler/contact_detail.go b/internal/api/handler/contact_detail.go
index 09f0ed3e..73beb7da 100644
--- a/internal/api/handler/contact_detail.go
+++ b/internal/api/handler/contact_detail.go
@@ -180,6 +180,10 @@ func (h *Handler) ListContactTimeline(c *gin.Context) {
 			errx.Handle(c, xerr)
 			return
 		}
+		if !models.ContactTimelineSource(source).Valid() {
+			errx.Handle(c, errx.New(errx.BadRequest, "invalid cursor"))
+			return
+		}
 		cursor = &models.ContactTimelineKey{At: at, Source: models.ContactTimelineSource(source), ID: id}
 	} else if raw := c.Query("before"); raw != "" {
 		t, err := time.Parse(time.RFC3339Nano, raw)
diff --git a/internal/models/contact.go b/internal/models/contact.go
index 1c514ef8..8ef9d7c7 100644
--- a/internal/models/contact.go
+++ b/internal/models/contact.go
@@ -440,6 +440,14 @@ const (
 	TimelineSourcePageHit        ContactTimelineSource = 14 // website_page_hits
 )
 
+// Valid reports whether s names a source the timeline is merged from. A
+// cursor carrying any other rank is malformed: zero is reserved for the
+// legacy bare-timestamp bound and anything above the last source would
+// re-admit the events at the cursor's instant.
+func (s ContactTimelineSource) Valid() bool {
+	return s >= TimelineSourceProgressSent && s <= TimelineSourcePageHit
+}
+
 // ContactTimelineKey is one event's position in the merged feed. A page
 // resumes strictly after the key of the last event it returned, comparing
 // (At, Source, ID) as a tuple, which is what the opaque cursor carries.
diff --git a/internal/models/contact_timeline_test.go b/internal/models/contact_timeline_test.go
new file mode 100644
index 00000000..231d7182
--- /dev/null
+++ b/internal/models/contact_timeline_test.go
@@ -0,0 +1,16 @@
+package models
+
+import "testing"
+
+func TestContactTimelineSourceValid(t *testing.T) {
+	for s := TimelineSourceProgressSent; s <= TimelineSourcePageHit; s++ {
+		if !s.Valid() {
+			t.Fatalf("source %d is one the feed merges and must be valid", s)
+		}
+	}
+	for _, s := range []ContactTimelineSource{0, -1, TimelineSourcePageHit + 1, 99} {
+		if s.Valid() {
+			t.Fatalf("source %d names no table and must be rejected in a cursor", s)
+		}
+	}
+}