diff --git a/panda/src/gobj/geomVertexReader.I b/panda/src/gobj/geomVertexReader.I
index 544bfea79e..1355f3f48b 100644
--- a/panda/src/gobj/geomVertexReader.I
+++ b/panda/src/gobj/geomVertexReader.I
@@ -607,7 +607,10 @@ quick_set_pointer(int row) {
   _pointer = _pointer_begin + _packer->_column->get_start() + _stride * row;
 
 #if defined(_DEBUG)
-  nassertv(_pointer_begin == _pointer_end || _pointer < _pointer_end);
+  // We have to allow the pointer to exceed the end by up to one row's
+  // width.  This wouldn't be legal on a plain GeomVertexReader, but
+  // it *is* legal for a GeomVertexRewriter.
+  nassertv(_pointer_begin == _pointer_end || (_pointer - _packer->_column->get_start())  <= _pointer_end);
 #endif
 }
 
diff --git a/panda/src/gobj/geomVertexWriter.I b/panda/src/gobj/geomVertexWriter.I
index 362c09034e..7f8c8732ba 100644
--- a/panda/src/gobj/geomVertexWriter.I
+++ b/panda/src/gobj/geomVertexWriter.I
@@ -860,7 +860,9 @@ quick_set_pointer(int row) {
   _pointer = _pointer_begin + _packer->_column->get_start() + _stride * row;
 
 #if defined(_DEBUG)
-  nassertv(_pointer_begin == _pointer_end || _pointer <= _pointer_end);
+  // We have to allow the pointer to exceed the end by up to one row's
+  // width, because the next call might be to add_data_*().
+  nassertv(_pointer_begin == _pointer_end || (_pointer - _packer->_column->get_start())  <= _pointer_end);
 #endif
 }