diff --git a/tutorial/01-visualizing.ipynb b/tutorial/01-visualizing.ipynb
index 70bea240..963037f7 100644
--- a/tutorial/01-visualizing.ipynb
+++ b/tutorial/01-visualizing.ipynb
@@ -44,7 +44,10 @@
    "cell_type": "code",
    "execution_count": 1,
    "metadata": {
-    "collapsed": true
+    "collapsed": true,
+    "jupyter": {
+     "outputs_hidden": true
+    }
    },
    "outputs": [
     {
@@ -111,7 +114,10 @@
    "cell_type": "code",
    "execution_count": 3,
    "metadata": {
-    "collapsed": true
+    "collapsed": true,
+    "jupyter": {
+     "outputs_hidden": true
+    }
    },
    "outputs": [
     {
@@ -532,7 +538,7 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 3",
+   "display_name": "Python 3 (ipykernel)",
    "language": "python",
    "name": "python3"
   },
@@ -546,7 +552,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.6.9"
+   "version": "3.13.3"
   }
  },
  "nbformat": 4,
diff --git a/udapi/block/corefud/mergesamespan.py b/udapi/block/corefud/mergesamespan.py
index 61b613cb..fdab35e1 100644
--- a/udapi/block/corefud/mergesamespan.py
+++ b/udapi/block/corefud/mergesamespan.py
@@ -50,3 +50,8 @@ def process_tree(self, tree):
                 mB.entity.mentions.remove(mB)
             except ValueError:
                 pass
+
+            # This is a sentence level block so it should not remove an entity at Doc level,
+            # but we can at least warn if this creates an empty entity
+            if len(mB.entity.mentions) == 0:
+                logging.warning(f"Entity now contains no mentions after removal of mention: {mB.entity.eid} entity of {mB}")