diff --git a/docs/changelog/125716.yaml b/docs/changelog/125716.yaml
new file mode 100644
index 0000000000000..fa85812c72c1a
--- /dev/null
+++ b/docs/changelog/125716.yaml
@@ -0,0 +1,5 @@
+pr: 125716
+summary: Return appropriate error on null dims update instead of npe
+area: Vector Search
+type: bug
+issues: []
diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/40_knn_search.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/40_knn_search.yml
index 9cb17af41dfbf..481f7879e0c4b 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/40_knn_search.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/40_knn_search.yml
@@ -596,3 +596,28 @@ setup:
   - match: { hits.hits.0._score: $knn_score0 }
   - match: { hits.hits.1._score: $knn_score1 }
   - match: { hits.hits.2._score: $knn_score2 }
+---
+"Updating dim to null is not allowed":
+  - requires:
+      cluster_features: "mapper.npe_on_dims_update_fix"
+      reason: "dims update fix"
+  - do:
+      indices.create:
+        index: test_index
+
+  - do:
+      indices.put_mapping:
+        index: test_index
+        body:
+          properties:
+            embedding:
+              type: dense_vector
+              dims: 4
+  - do:
+      catch: bad_request
+      indices.put_mapping:
+        index: test_index
+        body:
+          properties:
+            embedding:
+              type: dense_vector
diff --git a/server/src/main/java/org/elasticsearch/index/mapper/MapperFeatures.java b/server/src/main/java/org/elasticsearch/index/mapper/MapperFeatures.java
index d4514e40a2df6..f1f4f9b8faecd 100644
--- a/server/src/main/java/org/elasticsearch/index/mapper/MapperFeatures.java
+++ b/server/src/main/java/org/elasticsearch/index/mapper/MapperFeatures.java
@@ -67,6 +67,7 @@ public Set<NodeFeature> getFeatures() {
     static final NodeFeature UKNOWN_FIELD_MAPPING_UPDATE_ERROR_MESSAGE = new NodeFeature(
         "mapper.unknown_field_mapping_update_error_message"
     );
+    static final NodeFeature NPE_ON_DIMS_UPDATE_FIX = new NodeFeature("mapper.npe_on_dims_update_fix");
 
     @Override
     public Set<NodeFeature> getTestFeatures() {
@@ -88,7 +89,8 @@ public Set<NodeFeature> getTestFeatures() {
             SourceFieldMapper.SYNTHETIC_RECOVERY_SOURCE,
             ObjectMapper.SUBOBJECTS_FALSE_MAPPING_UPDATE_FIX,
             UKNOWN_FIELD_MAPPING_UPDATE_ERROR_MESSAGE,
-            DateFieldMapper.INVALID_DATE_FIX
+            DateFieldMapper.INVALID_DATE_FIX,
+            NPE_ON_DIMS_UPDATE_FIX
         );
     }
 }
diff --git a/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java b/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java
index 4a44ad2f360df..251998c84b8b7 100644
--- a/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java
+++ b/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java
@@ -147,7 +147,7 @@ public static class Builder extends FieldMapper.Builder {
             }
 
             return XContentMapValues.nodeIntegerValue(o);
-        }, m -> toType(m).fieldType().dims, XContentBuilder::field, Object::toString).setSerializerCheck((id, ic, v) -> v != null)
+        }, m -> toType(m).fieldType().dims, XContentBuilder::field, Objects::toString).setSerializerCheck((id, ic, v) -> v != null)
             .setMergeValidator((previous, current, c) -> previous == null || Objects.equals(previous, current))
             .addValidator(dims -> {
                 if (dims == null) {
diff --git a/x-pack/plugin/rank-vectors/src/main/java/org/elasticsearch/xpack/rank/vectors/mapper/RankVectorsFieldMapper.java b/x-pack/plugin/rank-vectors/src/main/java/org/elasticsearch/xpack/rank/vectors/mapper/RankVectorsFieldMapper.java
index a595eedaf4b8d..2686ab9c0b016 100644
--- a/x-pack/plugin/rank-vectors/src/main/java/org/elasticsearch/xpack/rank/vectors/mapper/RankVectorsFieldMapper.java
+++ b/x-pack/plugin/rank-vectors/src/main/java/org/elasticsearch/xpack/rank/vectors/mapper/RankVectorsFieldMapper.java
@@ -89,7 +89,7 @@ public static class Builder extends FieldMapper.Builder {
             }
 
             return XContentMapValues.nodeIntegerValue(o);
-        }, m -> toType(m).fieldType().dims, XContentBuilder::field, Object::toString).setSerializerCheck((id, ic, v) -> v != null)
+        }, m -> toType(m).fieldType().dims, XContentBuilder::field, Objects::toString).setSerializerCheck((id, ic, v) -> v != null)
             .setMergeValidator((previous, current, c) -> previous == null || Objects.equals(previous, current))
             .addValidator(dims -> {
                 if (dims == null) {
diff --git a/x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/rank_vectors/rank_vectors.yml b/x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/rank_vectors/rank_vectors.yml
index 791712ee925a5..837afecb3625b 100644
--- a/x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/rank_vectors/rank_vectors.yml
+++ b/x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/rank_vectors/rank_vectors.yml
@@ -135,3 +135,28 @@ setup:
         id: "1"
         body:
           vector1: [[2, -1, 1], [[2, -1, 1]]]
+---
+"Updating dim to null is not allowed":
+  - requires:
+      cluster_features: "mapper.npe_on_dims_update_fix"
+      reason: "dims update fix"
+  - do:
+      indices.create:
+        index: test_index
+
+  - do:
+      indices.put_mapping:
+        index: test_index
+        body:
+          properties:
+            embedding:
+              type: rank_vectors
+              dims: 4
+  - do:
+      catch: bad_request
+      indices.put_mapping:
+        index: test_index
+        body:
+          properties:
+            embedding:
+              type: rank_vectors