diff --git a/tools/db_crashtest.py b/tools/db_crashtest.py
index a10ee6c24..3fab88d88 100644
--- a/tools/db_crashtest.py
+++ b/tools/db_crashtest.py
@@ -134,6 +134,7 @@ def finalize_and_sanitize(src_params):
     # --allow_concurrent_memtable_write with --filter_deletes is not supported.
     if dest_params.get("allow_concurrent_memtable_write", 1) == 1:
         dest_params["filter_deletes"] = 0
+        dest_params["memtablerep"] = "skip_list"
     return dest_params
 
 
diff --git a/tools/db_stress.cc b/tools/db_stress.cc
index 10193a182..8e0f9796a 100644
--- a/tools/db_stress.cc
+++ b/tools/db_stress.cc
@@ -2008,11 +2008,17 @@ class StressTest {
         FLAGS_allow_concurrent_memtable_write;
     options_.enable_write_thread_adaptive_yield =
         FLAGS_enable_write_thread_adaptive_yield;
-    if ((FLAGS_prefix_size == 0) == (FLAGS_rep_factory == kHashSkipList)) {
+
+    if (FLAGS_prefix_size == 0 && FLAGS_rep_factory == kHashSkipList) {
       fprintf(stderr,
-            "prefix_size should be non-zero iff memtablerep == prefix_hash\n");
+              "prefeix_size cannot be zero if memtablerep == prefix_hash\n");
       exit(1);
     }
+    if (FLAGS_prefix_size != 0 && FLAGS_rep_factory != kHashSkipList) {
+      fprintf(stderr,
+              "WARNING: prefix_size is non-zero but "
+              "memtablerep != prefix_hash\n");
+    }
     switch (FLAGS_rep_factory) {
       case kSkipList:
         // no need to do anything