diff --git a/scripts/build-ucs-satellites.py b/scripts/build-ucs-satellites.py
index 62e29be93..86fc9063e 100644
--- a/scripts/build-ucs-satellites.py
+++ b/scripts/build-ucs-satellites.py
@@ -62,6 +62,9 @@ def build(xlsx: Path) -> dict:
         except (TypeError, ValueError):
             invalid_norad += 1
             continue
+        if norad <= 0:
+            invalid_norad += 1
+            continue
         record = [norad]
         for key, column in COLUMNS.items():
             text = clean(row[header[column]])
diff --git a/src/data/local_data/ucs_satellites/README.md b/src/data/local_data/ucs_satellites/README.md
index 8bd77ab45..1cd3c63f8 100644
--- a/src/data/local_data/ucs_satellites/README.md
+++ b/src/data/local_data/ucs_satellites/README.md
@@ -42,9 +42,9 @@ DATA_SOURCES.md.
   `0` meaning empty.
 - The 3,996 Starlink rows are dropped: every Starlink object is classed and
   credited to SpaceX by name, so they would only add size.
-- Rows without a readable NORAD number are dropped and counted apart from
-  duplicate NORAD numbers (the first row of a duplicate is kept); both counts
-  are 0 in this release.
+- Rows without a readable, positive NORAD number are dropped and counted apart
+  from duplicate NORAD numbers (the first row of a duplicate is kept); both
+  counts are 0 in this release.
 - Owner countries stay as UCS writes them ("USA", "USA/Japan", "ESA",
   "Multinational"); `src/layers/satellites/classes.js` normalises them, together with
   CelesTrak SATCAT owner codes, in one table.