diff --git a/rsl/eval.py b/rsl/eval.py
index 501f5ee..f038dc0 100644
--- a/rsl/eval.py
+++ b/rsl/eval.py
@@ -260,6 +260,9 @@ def accept_BinaryOpNode(self, node):
             
         if self.runtime.is_set(lhs):
             rhs = self.runtime.cast_to_set(rhs)
+
+        if self.runtime.is_set(rhs):
+            lhs = self.runtime.cast_to_set(lhs)
             
         value = ops[node.sign](lhs, rhs)
         
diff --git a/tests/test_binop.py b/tests/test_binop.py
index 90c273d..361f0e7 100644
--- a/tests/test_binop.py
+++ b/tests/test_binop.py
@@ -254,3 +254,43 @@ def test_instance_minus_same_instance(self):
         rc = self.eval_text(text)
         self.assertEqual(0, rc)
         
+
+    def test_instance_set_minus_instance_set(self):
+        self.metamodel.define_class('A', [('Name', 'string')])
+
+        text = '''
+        .create object instance a1 of A
+        .create object instance a2 of A
+        .assign a1.Name = "A1"
+        .assign a2.Name = "A2"
+        .select many an_set from instances of A
+        .select many a0_set from instances of A where (false)
+
+        .if ((cardinality (an_set - a0_set)) != 2)
+          .exit 1
+        .end if
+
+        .if ((cardinality (a0_set - an_set)) != 0)
+          .exit 2
+        .end if
+
+        .if ((cardinality (an_set - a1)) != 1)
+          .exit 3
+        .end if
+
+        .if ((cardinality (a1 - an_set)) != 0)
+          .exit 4
+        .end if
+
+        .if ((cardinality (a0_set - a1)) != 0)
+          .exit 5
+        .end if
+
+        .if ((cardinality (a1 - a0_set)) != 1)
+          .exit 6
+        .end if
+
+        .exit 0
+        '''
+        rc = self.eval_text(text)
+        self.assertEqual(0, rc)