diff --git a/docs/changelog/108459.yaml b/docs/changelog/108459.yaml
new file mode 100644
index 0000000000000..5e05797f284be
--- /dev/null
+++ b/docs/changelog/108459.yaml
@@ -0,0 +1,6 @@
+pr: 108459
+summary: Do not use global ordinals strategy if the leaf reader context cannot be
+  obtained
+area: Machine Learning
+type: bug
+issues: []
diff --git a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/FrequentItemSetCollector.java b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/FrequentItemSetCollector.java
index 18086748d6fe0..bd80e362f2f71 100644
--- a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/FrequentItemSetCollector.java
+++ b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/FrequentItemSetCollector.java
@@ -177,7 +177,8 @@ FrequentItemSet toFrequentItemSet(List<Field> fields) throws IOException {
             int pos = items.nextSetBit(0);
             while (pos > 0) {
                 Tuple<Integer, Object> item = transactionStore.getItem(topItemIds.getItemIdAt(pos - 1));
-                assert item.v1() < fields.size() : "item id exceed number of given items, did you configure eclat correctly?";
+                assert item.v1() < fields.size()
+                    : "eclat error: item id (" + item.v1() + ") exceeds the number of given items (" + fields.size() + ")";
                 final Field field = fields.get(item.v1());
                 Object formattedValue = field.formatValue(item.v2());
                 String fieldName = fields.get(item.v1()).getName();
@@ -252,19 +253,20 @@ public FrequentItemSetCollector(TransactionStore transactionStore, TopItemIds to
         this.topItemIds = topItemIds;
         this.size = size;
         this.min = min;
-        queue = new FrequentItemSetPriorityQueue(size);
-        frequentItemsByCount = Maps.newMapWithExpectedSize(size / 10);
+        this.queue = new FrequentItemSetPriorityQueue(size);
+        this.frequentItemsByCount = Maps.newMapWithExpectedSize(size / 10);
     }
 
     public FrequentItemSet[] finalizeAndGetResults(List<Field> fields) throws IOException {
-        FrequentItemSet[] topFrequentItems = new FrequentItemSet[size()];
+        FrequentItemSet[] topFrequentItems = new FrequentItemSet[queue.size()];
         for (int i = topFrequentItems.length - 1; i >= 0; i--) {
             topFrequentItems[i] = queue.pop().toFrequentItemSet(fields);
         }
         return topFrequentItems;
     }
 
-    public int size() {
+    // Visible for testing
+    int size() {
         return queue.size();
     }
 
diff --git a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/mr/ItemSetMapReduceAggregator.java b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/mr/ItemSetMapReduceAggregator.java
index 72bfb6f1f0394..0f9555c77341f 100644
--- a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/mr/ItemSetMapReduceAggregator.java
+++ b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/mr/ItemSetMapReduceAggregator.java
@@ -86,17 +86,15 @@ protected ItemSetMapReduceAggregator(
 
         boolean rewriteBasedOnOrdinals = false;
 
-        if (ctx.isPresent()) {
-            for (var c : configsAndValueFilters) {
-                ItemSetMapReduceValueSource e = context.getValuesSourceRegistry()
-                    .getAggregator(registryKey, c.v1())
-                    .build(c.v1(), id++, c.v2(), ordinalOptimization, ctx.get());
-                if (e.getField().getName() != null) {
-                    fields.add(e.getField());
-                    valueSources.add(e);
-                }
-                rewriteBasedOnOrdinals |= e.usesOrdinals();
+        for (var c : configsAndValueFilters) {
+            ItemSetMapReduceValueSource e = context.getValuesSourceRegistry()
+                .getAggregator(registryKey, c.v1())
+                .build(c.v1(), id++, c.v2(), ordinalOptimization, ctx);
+            if (e.getField().getName() != null) {
+                fields.add(e.getField());
+                valueSources.add(e);
             }
+            rewriteBasedOnOrdinals |= e.usesOrdinals();
         }
 
         this.rewriteBasedOnOrdinals = rewriteBasedOnOrdinals;
diff --git a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/mr/ItemSetMapReduceValueSource.java b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/mr/ItemSetMapReduceValueSource.java
index c9ec772eb3321..08adecd3fbce5 100644
--- a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/mr/ItemSetMapReduceValueSource.java
+++ b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/frequentitemsets/mr/ItemSetMapReduceValueSource.java
@@ -37,6 +37,7 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
+import java.util.Optional;
 
 /**
  * Interface to extract values from Lucene in order to feed it into the MapReducer.
@@ -53,7 +54,7 @@ ItemSetMapReduceValueSource build(
             int id,
             IncludeExclude includeExclude,
             AbstractItemSetMapReducer.OrdinalOptimization ordinalOptimization,
-            LeafReaderContext ctx
+            Optional<LeafReaderContext> ctx
         ) throws IOException;
     }
 
@@ -345,20 +346,21 @@ public KeywordValueSource(
             int id,
             IncludeExclude includeExclude,
             AbstractItemSetMapReducer.OrdinalOptimization ordinalOptimization,
-            LeafReaderContext ctx
+            Optional<LeafReaderContext> ctx
         ) throws IOException {
             super(config, id, ValueFormatter.BYTES_REF);
 
             if (AbstractItemSetMapReducer.OrdinalOptimization.GLOBAL_ORDINALS.equals(ordinalOptimization)
                 && config.getValuesSource() instanceof Bytes.WithOrdinals
-                && ((Bytes.WithOrdinals) config.getValuesSource()).supportsGlobalOrdinalsMapping()) {
+                && ((Bytes.WithOrdinals) config.getValuesSource()).supportsGlobalOrdinalsMapping()
+                && ctx.isPresent()) {
                 logger.debug("Use ordinals for field [{}]", config.fieldContext().field());
 
                 this.executionStrategy = new GlobalOrdinalsStrategy(
                     getField(),
                     (Bytes.WithOrdinals) config.getValuesSource(),
                     includeExclude == null ? null : includeExclude.convertToOrdinalsFilter(config.format()),
-                    ctx
+                    ctx.get()
                 );
             } else {
                 this.executionStrategy = new MapStrategy(
@@ -394,7 +396,7 @@ public NumericValueSource(
             int id,
             IncludeExclude includeExclude,
             AbstractItemSetMapReducer.OrdinalOptimization unusedOrdinalOptimization,
-            LeafReaderContext unusedCtx
+            Optional<LeafReaderContext> unusedCtx
         ) {
             super(config, id, ValueFormatter.LONG);
             this.source = (Numeric) config.getValuesSource();