diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8f77d9c..d8cc210 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ## [Unreleased]
 
 * Add `DB::cancel_all_background_work` method (stanislav-tkach)
+* Bump `librocksdb-sys` up to 6.13.3 (aleksuss)
 
 ## 0.15.0 (2020-08-25)
 
diff --git a/librocksdb-sys/rocksdb_lib_sources.txt b/librocksdb-sys/rocksdb_lib_sources.txt
index 9ebf882..bf06ae2 100644
--- a/librocksdb-sys/rocksdb_lib_sources.txt
+++ b/librocksdb-sys/rocksdb_lib_sources.txt
@@ -244,4 +244,4 @@ utilities/transactions/write_unprepared_txn.cc
 utilities/transactions/write_unprepared_txn_db.cc
 utilities/ttl/db_ttl_impl.cc
 utilities/write_batch_with_index/write_batch_with_index.cc
-utilities/write_batch_with_index/write_batch_with_index_internal.cc
+utilities/write_batch_with_index/write_batch_with_index_internal.cc
\ No newline at end of file
diff --git a/librocksdb-sys/tests/ffi.rs b/librocksdb-sys/tests/ffi.rs
index 84f8688..07419ba 100644
--- a/librocksdb-sys/tests/ffi.rs
+++ b/librocksdb-sys/tests/ffi.rs
@@ -89,21 +89,25 @@ macro_rules! CheckCondition {
     };
 }
 
-unsafe fn CheckEqual(expected: *const c_char, v: *const c_char, n: size_t) {
-    if expected.is_null() && v.is_null() {
-        // ok
+unsafe fn CheckEqual(expected: *const c_char, actual: *const c_char, n: size_t) {
+    let is_equal = if expected.is_null() && actual.is_null() {
+        true
     } else if !expected.is_null()
-        && !v.is_null()
+        && !actual.is_null()
         && n == strlen(expected)
-        && memcmp(expected as *const c_void, v as *const c_void, n) == 0
+        && memcmp(expected as *const c_void, actual as *const c_void, n) == 0
     {
-        // ok
+        true
     } else {
+        false
+    };
+
+    if !is_equal {
         panic!(
             "{}: expected '{}', got '{}'",
             phase,
             rstr(strndup(expected, n)),
-            rstr(strndup(v, 5))
+            rstr(strndup(actual, 5))
         );
     }
 }
diff --git a/src/merge_operator.rs b/src/merge_operator.rs
index e7d4b6f..bdee01a 100644
--- a/src/merge_operator.rs
+++ b/src/merge_operator.rs
@@ -114,15 +114,18 @@ pub unsafe extern "C" fn full_merge_callback(
             existing_value_len as usize,
         ))
     };
-    if let Some(result) = (cb.full_merge_fn)(key, oldval, operands) {
-        *new_value_length = result.len() as size_t;
-        *success = 1_u8;
-        Box::into_raw(result.into_boxed_slice()) as *mut c_char
-    } else {
-        *new_value_length = 0;
-        *success = 0_u8;
-        ptr::null_mut() as *mut c_char
-    }
+    (cb.full_merge_fn)(key, oldval, operands).map_or_else(
+        || {
+            *new_value_length = 0;
+            *success = 0_u8;
+            ptr::null_mut() as *mut c_char
+        },
+        |result| {
+            *new_value_length = result.len() as size_t;
+            *success = 1_u8;
+            Box::into_raw(result.into_boxed_slice()) as *mut c_char
+        },
+    )
 }
 
 pub unsafe extern "C" fn partial_merge_callback(
@@ -138,15 +141,18 @@ pub unsafe extern "C" fn partial_merge_callback(
     let cb = &mut *(raw_cb as *mut MergeOperatorCallback);
     let operands = &mut MergeOperands::new(operands_list, operands_list_len, num_operands);
     let key = slice::from_raw_parts(raw_key as *const u8, key_len as usize);
-    if let Some(result) = (cb.partial_merge_fn)(key, None, operands) {
-        *new_value_length = result.len() as size_t;
-        *success = 1_u8;
-        Box::into_raw(result.into_boxed_slice()) as *mut c_char
-    } else {
-        *new_value_length = 0;
-        *success = 0_u8;
-        ptr::null_mut::<c_char>()
-    }
+    (cb.partial_merge_fn)(key, None, operands).map_or_else(
+        || {
+            *new_value_length = 0;
+            *success = 0_u8;
+            ptr::null_mut::<c_char>()
+        },
+        |result| {
+            *new_value_length = result.len() as size_t;
+            *success = 1_u8;
+            Box::into_raw(result.into_boxed_slice()) as *mut c_char
+        },
+    )
 }
 
 pub struct MergeOperands {