diff --git a/server/src/main/java/org/elasticsearch/index/codec/tsdb/es819/DocValuesConsumerUtil.java b/server/src/main/java/org/elasticsearch/index/codec/tsdb/es819/DocValuesConsumerUtil.java
index 9f9dbd11f52a3..4e9bf9c09a3b0 100644
--- a/server/src/main/java/org/elasticsearch/index/codec/tsdb/es819/DocValuesConsumerUtil.java
+++ b/server/src/main/java/org/elasticsearch/index/codec/tsdb/es819/DocValuesConsumerUtil.java
@@ -64,6 +64,9 @@ static MergeStats compatibleWithOptimizedMerge(boolean optimizedMergeEnabled, Me
                             if (entry != null) {
                                 sumNumValues += entry.numValues;
                                 sumNumDocsWithField += entry.numDocsWithField;
+                            } else {
+                                assert false : "unexpectedly got no entry for field [" + fieldInfo.number + "\\" + fieldInfo.name + "]";
+                                return UNSUPPORTED;
                             }
                         }
                         case SORTED_NUMERIC -> {
@@ -71,6 +74,9 @@ static MergeStats compatibleWithOptimizedMerge(boolean optimizedMergeEnabled, Me
                             if (entry != null) {
                                 sumNumValues += entry.numValues;
                                 sumNumDocsWithField += entry.numDocsWithField;
+                            } else {
+                                assert false : "unexpectedly got no entry for field [" + fieldInfo.number + "\\" + fieldInfo.name + "]";
+                                return UNSUPPORTED;
                             }
                         }
                         case SORTED -> {
@@ -78,6 +84,9 @@ static MergeStats compatibleWithOptimizedMerge(boolean optimizedMergeEnabled, Me
                             if (entry != null) {
                                 sumNumValues += entry.ordsEntry.numValues;
                                 sumNumDocsWithField += entry.ordsEntry.numDocsWithField;
+                            } else {
+                                assert false : "unexpectedly got no entry for field [" + fieldInfo.number + "\\" + fieldInfo.name + "]";
+                                return UNSUPPORTED;
                             }
                         }
                         case SORTED_SET -> {
@@ -90,6 +99,9 @@ static MergeStats compatibleWithOptimizedMerge(boolean optimizedMergeEnabled, Me
                                     sumNumValues += entry.ordsEntry.numValues;
                                     sumNumDocsWithField += entry.ordsEntry.numDocsWithField;
                                 }
+                            } else {
+                                assert false : "unexpectedly got no entry for field [" + fieldInfo.number + "\\" + fieldInfo.name + "]";
+                                return UNSUPPORTED;
                             }
                         }
                         case BINARY -> {
@@ -98,6 +110,9 @@ static MergeStats compatibleWithOptimizedMerge(boolean optimizedMergeEnabled, Me
                                 sumNumDocsWithField += entry.numDocsWithField;
                                 minLength = Math.min(minLength, entry.minLength);
                                 maxLength = Math.max(maxLength, entry.maxLength);
+                            } else {
+                                assert false : "unexpectedly got no entry for field [" + fieldInfo.number + "\\" + fieldInfo.name + "]";
+                                return UNSUPPORTED;
                             }
                         }
                         default -> throw new IllegalStateException("unexpected doc values producer type: " + fieldInfo.getDocValuesType());