diff --git a/src/common/algorithm.c b/src/common/algorithm.c
index 55a7a927..1d907be3 100644
--- a/src/common/algorithm.c
+++ b/src/common/algorithm.c
@@ -12,10 +12,10 @@ static bool _stable_sort_merge(char* a,
                   int (*f_lt)(const void* a, const void* b, void* extra),
                   void* extra) {
     while(a < a_end && b < b_end) {
-        int res = f_lt(a, b, extra);
+        int res = f_lt(b, a, extra);
         // check error
         if(res == -1) return false;
-        if(res) {
+        if(res == 0) {  // !(b<a) -> (a>=b)
             memcpy(r, a, elem_size);
             a += elem_size;
         } else {
diff --git a/tests/05_list.py b/tests/05_list.py
index 5f6b213c..cae71041 100644
--- a/tests/05_list.py
+++ b/tests/05_list.py
@@ -88,7 +88,7 @@ assert list(range(5, 1, -2)) == [5, 3]
 # test sort
 a = [8, 2, 4, 2, 9]
 assert a.sort() == None
-assert a == [2, 2, 4, 8, 9]
+assert (a == [2, 2, 4, 8, 9]), a
 
 a = []
 assert a.sort() == None
diff --git a/tests/95_bugs.py b/tests/95_bugs.py
index 63802be0..1916c34d 100644
--- a/tests/95_bugs.py
+++ b/tests/95_bugs.py
@@ -142,4 +142,10 @@ assert a.__new__ == list.__new__
 class A:
     x: list[int] = [i for i in range(1, 4)]
 
-assert A.x == [1, 2, 3]
\ No newline at end of file
+assert A.x == [1, 2, 3]
+
+# stable sort
+a = [(0, 1), (1, 1), (1, 2)]
+b = sorted(a, key=lambda x: x[0])
+if b != [(0, 1), (1, 1), (1, 2)]:
+    assert False, b
\ No newline at end of file