diff --git a/panda/src/gobj/geomVertexReader.I b/panda/src/gobj/geomVertexReader.I
index 893d6458cf..2fc27b911a 100644
--- a/panda/src/gobj/geomVertexReader.I
+++ b/panda/src/gobj/geomVertexReader.I
@@ -328,6 +328,7 @@ set_row_unsafe(int row) {
  */
 INLINE void GeomVertexReader::
 set_row(int row) {
+  nassertv(row >= 0);
   _start_row = row;
   if (has_column()) {
     bool result = set_pointer(_start_row);
diff --git a/panda/src/gobj/geomVertexWriter.I b/panda/src/gobj/geomVertexWriter.I
index b55fdd5f4a..00b9652e8d 100644
--- a/panda/src/gobj/geomVertexWriter.I
+++ b/panda/src/gobj/geomVertexWriter.I
@@ -297,6 +297,7 @@ set_row_unsafe(int row) {
  */
 INLINE void GeomVertexWriter::
 set_row(int row) {
+  nassertv(row >= 0);
   _start_row = row;
   if (has_column()) {
     set_pointer(_start_row);