diff --git a/src/include/common/hashfn_unstable.h b/src/include/common/hashfn_unstable.h
index 428936b8b64..f18cb4afe0a 100644
--- a/src/include/common/hashfn_unstable.h
+++ b/src/include/common/hashfn_unstable.h
@@ -86,7 +86,8 @@
  * By handling the terminator on-the-fly, we can avoid needing a strlen()
  * call to tell us how many bytes to hash. Experimentation has found that
  * SMHasher fails unless we incorporate the length, so it is passed to
- * the finalizer as a tweak.
+ * the finalizer as a tweak. When hashing multiple strings, it is
+ * recommended to pass the total length of the hashed strings.
  */
 
 
@@ -291,10 +292,13 @@ fasthash_accum_cstring(fasthash_state *hs, const char *str)
 #if SIZEOF_VOID_P >= 8
 
 	size_t		len;
-#ifdef USE_ASSERT_CHECKING
-	size_t		len_check;
-	fasthash_state hs_check;
+	size_t		len_check PG_USED_FOR_ASSERTS_ONLY;
+	fasthash_state hs_check PG_USED_FOR_ASSERTS_ONLY;
+
+	if (str == NULL)
+		return 0;
 
+#ifdef USE_ASSERT_CHECKING
 	memcpy(&hs_check, hs, sizeof(fasthash_state));
 	len_check = fasthash_accum_cstring_unaligned(&hs_check, str);
 #endif