diff --git a/lib/src/middleware/check_mirror_strands_legal.dart b/lib/src/middleware/check_mirror_strands_legal.dart
index e200d743f..55142195f 100644
--- a/lib/src/middleware/check_mirror_strands_legal.dart
+++ b/lib/src/middleware/check_mirror_strands_legal.dart
@@ -35,6 +35,7 @@ check_reflect_strands_legal_middleware(Store<AppState> store, action, NextDispat
     var altered_design = design.remove_strands(strands_to_reflect);
     altered_design = altered_design.add_strands(reflected_strands);
 
+    // check overlapping strands
     try {
       altered_design.check_strands_overlap_legally();
     } on IllegalDesignError catch (e) {
@@ -44,6 +45,17 @@ check_reflect_strands_legal_middleware(Store<AppState> store, action, NextDispat
       return;
     }
 
+    // check out of bounds helix
+    try {
+      altered_design.check_strands_in_bounds();
+    } on IllegalDesignError catch (e) {
+      var msg = 'Cannot mirror these strands ${action.horizontal ? "horizontally" : "vertically"}\n'
+          'Strands would go out of bounds:\n\n${e.cause}';
+      window.alert(msg);
+      return;
+    }
+    
+
     Map<int, Strand> new_strands = {};
     int idx_mirrored_strand = 0;
     for (var strand in strands_to_reflect) {
diff --git a/lib/src/state/design.dart b/lib/src/state/design.dart
index c00ea627e..f3f8c2cf5 100644
--- a/lib/src/state/design.dart
+++ b/lib/src/state/design.dart
@@ -1488,6 +1488,25 @@ abstract class Design with UnusedFields implements Built<Design, DesignBuilder>,
     }
   }
 
+  check_strands_in_bounds() {
+    String err_msg(Domain domain, int h_idx) {
+      return "domain found out of bounds on helix ${h_idx}: "
+          "\n${domain}";
+    }
+
+    for (int helix_idx in helices.keys) {
+      var domains = this.domains_on_helix(helix_idx);
+      var helix = helices[helix_idx];
+      if (domains.length == 0) continue;
+
+      for (var domain in domains) {
+        if (domain.start < helix.min_offset || domain.end > helix.max_offset) {
+              throw IllegalDesignError(err_msg(domain, helix_idx));
+            }
+      }
+    }
+  }
+
   @memoized
   BuiltMap<String, BuiltList<int>> get helix_idxs_in_group {
     Map<String, List<int>> map = {for (var name in groups.keys) name: []};
diff --git a/test/strand_mirror_unit_test.dart b/test/strand_mirror_unit_test.dart
new file mode 100644
index 000000000..cd7896236
--- /dev/null
+++ b/test/strand_mirror_unit_test.dart
@@ -0,0 +1,341 @@
+// @dart=2.9
+
+import 'dart:convert';
+
+import 'package:built_collection/built_collection.dart';
+import 'package:redux/redux.dart';
+import 'package:scadnano/src/actions/actions.dart';
+import 'package:scadnano/src/json_serializable.dart';
+import 'package:scadnano/src/reducers/app_state_reducer.dart';
+import 'package:scadnano/src/reducers/change_loopout_length.dart';
+import 'package:scadnano/src/reducers/delete_reducer.dart';
+import 'package:scadnano/src/reducers/nick_ligate_join_by_crossover_reducers.dart';
+import 'package:scadnano/src/reducers/assign_domain_names_reducer.dart';
+import 'package:scadnano/src/reducers/strands_reducer.dart';
+import 'package:scadnano/src/state/address.dart';
+import 'package:scadnano/src/state/app_state.dart';
+import 'package:scadnano/src/state/domain.dart';
+import 'package:scadnano/src/state/helix.dart';
+import 'package:scadnano/src/state/grid.dart';
+import 'package:scadnano/src/state/loopout.dart';
+import 'package:scadnano/src/state/select_mode.dart';
+import 'package:scadnano/src/state/selectable.dart';
+import 'package:scadnano/src/state/strand.dart';
+import 'package:scadnano/src/state/strands_move.dart';
+import 'package:test/test.dart';
+
+import 'package:scadnano/src/state/design.dart';
+import 'package:scadnano/src/actions/actions.dart' as actions;
+
+import 'utils.dart';
+
+main() {
+  
+  group('StrandReflectInvalid', () {
+    List<Helix> helices;
+    Design orig_design;
+
+    setUp(() {
+      /*
+
+      * initial design
+
+        0    5    10
+        |----|----|
+        
+      0 [---------\
+                  |
+                  |
+                  |
+      1           |
+             <----/   
+
+      */
+
+      helices = [
+        Helix(idx: 0, min_offset: 0, max_offset: 10, grid: Grid.square),
+        Helix(idx: 1, min_offset: 5, max_offset: 10, grid: Grid.square)
+      ];
+      orig_design = Design(helices: helices, grid: Grid.square);
+
+      orig_design = orig_design
+          .strand(0, 0)
+          .move(10)
+          .cross(1)
+          .move(-5)
+          .commit();
+    });
+
+    test('strand_reflect_horizontally_no_polarity_reverse', () {
+
+      Store<AppState> store = store_from_design(orig_design, initialize_app_instance: false);
+      store.dispatch(
+          actions.StrandsReflect(
+        strands: orig_design.strands, horizontal: true, reverse_polarity: false));
+        
+      /* 
+
+      * hypothetical reflect --- but notice domain on helix 1 is out of bounds! (min_offset is 5)
+      * therefore, reflect action should be cancelled
+
+        0    5    10
+        |----|----|
+        
+      0  
+        /---------]
+        |
+        |
+      1 \---->
+        
+      */
+      expect_design_equal(store.state.design, orig_design);
+    });
+
+    test('strand_reflect_vertically_no_polarity_reverse', () {
+
+      Store<AppState> store = store_from_design(orig_design, initialize_app_instance: false);
+      store.dispatch(
+          actions.StrandsReflect(
+        strands: orig_design.strands, horizontal: false, reverse_polarity: false));
+        
+      /* 
+
+      * hypothetical reflect --- but notice domain on helix 1 is out of bounds! (min_offset is 5)
+      * therefore, reflect action should be cancelled
+
+        0    5    10
+        |----|----|
+        
+      0      [----\
+                  |
+                  |
+                  |
+      1           |
+        <---------/   
+        
+      */
+      expect_design_equal(store.state.design, orig_design);
+    });
+
+    test('strand_reflect_horizontally_polarity_reverse', () {
+
+      Store<AppState> store = store_from_design(orig_design, initialize_app_instance: false);
+      store.dispatch(
+          actions.StrandsReflect(
+        strands: orig_design.strands, horizontal: true, reverse_polarity: true));
+        
+      /* 
+
+      * hypothetical reflect --- but notice domain on helix 1 is out of bounds! (min_offset is 5)
+      * therefore, reflect action should be cancelled
+
+        0    5    10
+        |----|----|
+        
+      0 /--------->
+        |
+        |
+        |
+      1 |
+        \----]
+        
+      */
+      expect_design_equal(store.state.design, orig_design);
+    });
+
+    test('strand_reflect_vertically_polarity_reverse', () {
+
+      Store<AppState> store = store_from_design(orig_design, initialize_app_instance: false);
+      store.dispatch(
+          actions.StrandsReflect(
+        strands: orig_design.strands, horizontal: false, reverse_polarity: true));
+        
+      /* 
+
+      * hypothetical reflect --- but notice domain on helix 1 is out of bounds! (min_offset is 5)
+      * therefore, reflect action should be cancelled
+
+        0    5    10
+        |----|----|
+        
+      0
+             <----\
+                  |
+                  |
+      1 [---------/   
+        
+      */
+      expect_design_equal(store.state.design, orig_design);
+    });
+  });
+  
+  group('StrandReflectValid', () {
+    List<Helix> helices;
+    Design orig_design;
+
+    setUp(() {
+      /*
+
+      * initial design
+
+        0    5    10
+        |----|----|
+        
+      0 [---------\
+                  |
+                  |
+                  |
+      1           |
+             <----/   
+
+      */
+
+      helices = [
+        Helix(idx: 0, min_offset: 0, max_offset: 10, grid: Grid.square),
+        Helix(idx: 1, min_offset: 0, max_offset: 10, grid: Grid.square)
+      ];
+      orig_design = Design(helices: helices, grid: Grid.square);
+
+      orig_design = orig_design
+          .strand(0, 0)
+          .move(10)
+          .cross(1)
+          .move(-5)
+          .commit();
+    });
+
+    test('strand_reflect_horizontally_no_polarity_reverse', () {
+
+      Store<AppState> store = store_from_design(orig_design, initialize_app_instance: false);
+      store.dispatch(
+          actions.StrandsReflect(
+        strands: orig_design.strands, horizontal: true, reverse_polarity: false));
+        
+      /* 
+
+      * resulting design
+
+        0    5    10
+        |----|----|
+        
+      0  
+        /---------]
+        |
+        |
+      1 \---->
+        
+      */
+
+      var expected_design = Design(helices: helices, grid: Grid.square);
+
+      expected_design = expected_design
+          .strand(0, 10)
+          .move(-10)
+          .cross(1)
+          .move(5)
+          .commit();
+      expect_design_equal(store.state.design, expected_design);
+    });
+
+    test('strand_reflect_vertically_no_polarity_reverse', () {
+
+      Store<AppState> store = store_from_design(orig_design, initialize_app_instance: false);
+      store.dispatch(
+          actions.StrandsReflect(
+        strands: orig_design.strands, horizontal: false, reverse_polarity: false));
+        
+      /* 
+
+      * resulting design
+
+        0    5    10
+        |----|----|
+        
+      0      [----\
+                  |
+                  |
+                  |
+      1           |
+        <---------/   
+        
+      */
+
+      var expected_design = Design(helices: helices, grid: Grid.square);
+
+      expected_design = expected_design
+          .strand(0, 5)
+          .move(5)
+          .cross(1)
+          .move(-10)
+          .commit();
+      expect_design_equal(store.state.design, expected_design);
+    });
+
+    test('strand_reflect_horizontally_polarity_reverse', () {
+
+      Store<AppState> store = store_from_design(orig_design, initialize_app_instance: false);
+      store.dispatch(
+          actions.StrandsReflect(
+        strands: orig_design.strands, horizontal: true, reverse_polarity: true));
+        
+      /* 
+
+      * resulting design
+
+        0    5    10
+        |----|----|
+        
+      0 /--------->
+        |
+        |
+        |
+      1 |
+        \----]
+        
+      */
+
+      var expected_design = Design(helices: helices, grid: Grid.square);
+
+      expected_design = expected_design
+          .strand(1, 5)
+          .move(-5)
+          .cross(0)
+          .move(10)
+          .commit();
+      expect_design_equal(store.state.design, expected_design);
+    });
+
+    test('strand_reflect_vertically_polarity_reverse', () {
+
+      Store<AppState> store = store_from_design(orig_design, initialize_app_instance: false);
+      store.dispatch(
+          actions.StrandsReflect(
+        strands: orig_design.strands, horizontal: false, reverse_polarity: true));
+        
+      /* 
+
+      * resulting design
+
+        0    5    10
+        |----|----|
+        
+      0
+             <----\
+                  |
+                  |
+      1 [---------/   
+        
+      */
+
+      var expected_design = Design(helices: helices, grid: Grid.square);
+
+      expected_design = expected_design
+          .strand(1, 0)
+          .move(10)
+          .cross(0)
+          .move(-5)
+          .commit();
+      expect_design_equal(store.state.design, expected_design);
+    });
+  });
+}