diff --git a/mk.lua b/mk.lua
index f874240..9417c42 100644
--- a/mk.lua
+++ b/mk.lua
@@ -167,15 +167,22 @@ local function walk_all(v, s)
    end
 end
 
+local function occurs_check(var, term)
+   if not is_table(term) then return false end
+   if #term == 0 then return false end
+   if is_var(term) then return var == term end
+   return occurs_check(var, car(term)) or occurs_check(var, cdr(term))
+end
+
 local function unify(k, v, s)
    local k = walk(k, s)
    local v = walk(v, s)
    if k == v then
       return s
    elseif is_var(k) then
-      return cons(cons(k, v), s)
+      return (not occurs_check(k, v)) and cons(cons(k, v), s)
    elseif is_var(v) then
-      return cons(cons(v, k), s)
+      return (not occurs_check(v, k)) and cons(cons(v, k), s)
    elseif is_var(k) or is_var(v) then
       return cons(cons(k, v), s)
    elseif is_pair(v) and is_pair(k) then
diff --git a/test.lua b/test.lua
index f1cc459..44ba261 100644
--- a/test.lua
+++ b/test.lua
@@ -105,6 +105,10 @@ assert(equal(run(false, {a, b, c}, all(eq(a, 2), eq(a, b), not_eq(b, 3))), {{2,
 assert(equal(run(false, {a, b, c}, all(eq(c, 2), eq(a, b), not_eq(b, 3))), { { "_.0 not eq: 3", "_.0 not eq: 3", 2 } }))
 assert(equal(run(false, {a, b}, all(eq(a, b), not_eq(a, 3))), { { "_.0 not eq: 3", "_.0 not eq: 3" } }))
 
+assert(equal(run(1, a, eq(a, list(a, a))), {}))
+assert(equal(run(1, a, eq(a, list(a, b))), {}))
+assert(equal(run(1, a, eq(a, list(b, a))), {}))
+
 assert(equal(run(1, a, mergeo({1, {2, {3}}}, {4, {5, {6, {}}}}, a)), { list(1, 2, 3, 4, 5, 6) }))
 
 assert(equal(run(1, a, mergeo({1}, {2}, a)), { {1, {2}} }))