diff --git a/src/test/java/org/elasticsearch/search/functionscore/RandomScoreFunctionTests.java b/src/test/java/org/elasticsearch/search/functionscore/RandomScoreFunctionTests.java
index 0967035edf3..508669a8387 100644
--- a/src/test/java/org/elasticsearch/search/functionscore/RandomScoreFunctionTests.java
+++ b/src/test/java/org/elasticsearch/search/functionscore/RandomScoreFunctionTests.java
@@ -53,7 +53,11 @@ public class RandomScoreFunctionTests extends ElasticsearchIntegrationTest {
         int outerIters = scaledRandomIntBetween(10, 20);
         for (int o = 0; o < outerIters; o++) {
             final long seed = randomLong();
-            final String preference = randomRealisticUnicodeOfLengthBetween(1, 10); // at least one char!!
+            String preference = randomRealisticUnicodeOfLengthBetween(1, 10); // at least one char!!
+            // randomPreference should not start with '_' (reserved for known preference types (e.g. _shards, _primary)
+            while (preference.startsWith("_")) {
+                preference = randomRealisticUnicodeOfLengthBetween(1, 10);
+            }
             int innerIters = scaledRandomIntBetween(2, 5);
             SearchHits hits = null;
             for (int i = 0; i < innerIters; i++) {
diff --git a/src/test/java/org/elasticsearch/search/simple/SimpleSearchTests.java b/src/test/java/org/elasticsearch/search/simple/SimpleSearchTests.java
index edb739feb6a..f5dbe591d10 100644
--- a/src/test/java/org/elasticsearch/search/simple/SimpleSearchTests.java
+++ b/src/test/java/org/elasticsearch/search/simple/SimpleSearchTests.java
@@ -66,8 +66,13 @@ public class SimpleSearchTests extends ElasticsearchIntegrationTest {
 
         int iters = scaledRandomIntBetween(10, 20);
         for (int i = 0; i < iters; i++) {
+            String randomPreference = randomUnicodeOfLengthBetween(0, 4);
+            // randomPreference should not start with '_' (reserved for known preference types (e.g. _shards, _primary)
+            while (randomPreference.startsWith("_")) {
+                randomPreference = randomUnicodeOfLengthBetween(0, 4);
+            }
             // id is not indexed, but lets see that we automatically convert to
-            SearchResponse searchResponse = client().prepareSearch().setQuery(QueryBuilders.matchAllQuery()).setPreference(randomUnicodeOfLengthBetween(0, 4)).get();
+            SearchResponse searchResponse = client().prepareSearch().setQuery(QueryBuilders.matchAllQuery()).setPreference(randomPreference).get();
             assertHitCount(searchResponse, 6l);
 
         }