diff --git a/klippy/extras/bed_mesh.py b/klippy/extras/bed_mesh.py
index cb9e719c..1c25a1ea 100644
--- a/klippy/extras/bed_mesh.py
+++ b/klippy/extras/bed_mesh.py
@@ -404,6 +404,8 @@ class BedMeshCalibrate:
         if 'RELATIVE_REFERENCE_INDEX' in params:
             self.relative_reference_index = gcmd.get_int(
                 'RELATIVE_REFERENCE_INDEX')
+            if self.relative_reference_index < 0:
+                self.relative_reference_index = None
             need_cfg_update = True
         if self.radius is not None:
             if "MESH_RADIUS" in params: