diff --git a/src/blenderbim/blenderbim/bim/module/model/slab.py b/src/blenderbim/blenderbim/bim/module/model/slab.py
index fd20499814..ba6691799e 100644
--- a/src/blenderbim/blenderbim/bim/module/model/slab.py
+++ b/src/blenderbim/blenderbim/bim/module/model/slab.py
@@ -734,15 +734,20 @@ class SetArcIndex(bpy.types.Operator):
         return bool(obj) and obj.type == "MESH"
 
     def cancel_message(self, msg):
-        self.report({"WARNING"}, msg)
+        self.report({"ERROR"}, msg)
         return {"CANCELLED"}
 
     def execute(self, context):
         # NOTE: undo won't remove new verex group
         # because of jumping between modes
         obj = context.active_object
+
+        bm = tool.Blender.get_bmesh_for_mesh(obj.data)
+        selected_vertices = [v.index for v in bm.verts if v.select]
+        if len(selected_vertices) != 3:
+            return self.cancel_message("Select 3 vertices.")
+
         bpy.ops.object.mode_set(mode="OBJECT")
-        selected_vertices = [v.index for v in obj.data.vertices if v.select]
         in_group = any([bool(obj.data.vertices[v].groups) for v in selected_vertices])
         for group in obj.vertex_groups:
             group.remove(selected_vertices)