diff --git a/java/src/main/java/org/softwareheritage/graph/utils/ListOriginContributors.java b/java/src/main/java/org/softwareheritage/graph/utils/ListOriginContributors.java
--- a/java/src/main/java/org/softwareheritage/graph/utils/ListOriginContributors.java
+++ b/java/src/main/java/org/softwareheritage/graph/utils/ListOriginContributors.java
@@ -87,11 +87,21 @@
                 nodeContributors = new HashSet<>();
             }
 
+            Long personId;
             if (nodeSWHID.getType() == SwhType.REV) {
-                nodeContributors.add(underlyingGraph.getAuthorId(nodeId));
-                nodeContributors.add(underlyingGraph.getCommitterId(nodeId));
+                personId = underlyingGraph.getAuthorId(nodeId);
+                if (personId != null) {
+                    nodeContributors.add(personId);
+                }
+                personId = underlyingGraph.getCommitterId(nodeId);
+                if (personId != null) {
+                    nodeContributors.add(personId);
+                }
             } else if (nodeSWHID.getType() == SwhType.REL) {
-                nodeContributors.add(underlyingGraph.getAuthorId(nodeId));
+                personId = underlyingGraph.getAuthorId(nodeId);
+                if (personId != null) {
+                    nodeContributors.add(personId);
+                }
             }
 
             if (!reuseAncestorSet) {
diff --git a/swh/graph/tests/test_origin_contributors.py b/swh/graph/tests/test_origin_contributors.py
--- a/swh/graph/tests/test_origin_contributors.py
+++ b/swh/graph/tests/test_origin_contributors.py
@@ -33,7 +33,6 @@
 swh:1:ori:83404f995118bd25774f4ac14422a8f175e7a054,0
 swh:1:ori:83404f995118bd25774f4ac14422a8f175e7a054,2
 swh:1:ori:8f50d3f60eae370ddbf85c86219c55108a350165,0
-swh:1:ori:8f50d3f60eae370ddbf85c86219c55108a350165,null
 swh:1:ori:8f50d3f60eae370ddbf85c86219c55108a350165,1
 swh:1:ori:8f50d3f60eae370ddbf85c86219c55108a350165,2
 """