diff --git a/app/assets/javascripts/darkswarm/controllers/shop_variant_controller.js.coffee b/app/assets/javascripts/darkswarm/controllers/shop_variant_controller.js.coffee
index 311dbbc0bc..4b5b3e6a6e 100644
--- a/app/assets/javascripts/darkswarm/controllers/shop_variant_controller.js.coffee
+++ b/app/assets/javascripts/darkswarm/controllers/shop_variant_controller.js.coffee
@@ -17,21 +17,33 @@ Darkswarm.controller "ShopVariantCtrl", ($scope, $modal, Cart) ->
       if item.max_quantity < item.quantity
         item.quantity = item.max_quantity
 
+  $scope.quantity = ->
+    $scope.variant.line_item.quantity || 0
+
+  $scope.maxQuantity = ->
+    $scope.variant.line_item.max_quantity || $scope.sanitizedQuantity()
+
+  $scope.sanitizedQuantity = ->
+    Math.max(0, Math.min($scope.quantity(), $scope.available()))
+
+  $scope.sanitizedMaxQuantity = ->
+    Math.max($scope.sanitizedQuantity(), Math.min($scope.maxQuantity(), $scope.available()))
+
   $scope.add = (quantity) ->
     item = $scope.variant.line_item
-    item.quantity += quantity
+    item.quantity = $scope.sanitizedQuantity() + quantity
 
   $scope.addMax = (quantity) ->
     item = $scope.variant.line_item
-    item.max_quantity += quantity
+    item.max_quantity = $scope.sanitizedMaxQuantity() + quantity
 
   $scope.canAdd = (quantity) ->
-    wantedQuantity = $scope.variant.line_item.quantity + quantity
+    wantedQuantity = $scope.sanitizedQuantity() + quantity
     $scope.quantityValid(wantedQuantity)
 
   $scope.canAddMax = (quantity) ->
     variant = $scope.variant
-    wantedQuantity = variant.line_item.max_quantity + quantity
+    wantedQuantity = $scope.sanitizedMaxQuantity() + quantity
     $scope.quantityValid(wantedQuantity) && variant.line_item.quantity > 0
 
   $scope.available = ->
diff --git a/spec/javascripts/unit/darkswarm/controllers/checkout/shop_variant_controller_spec.js.coffee b/spec/javascripts/unit/darkswarm/controllers/checkout/shop_variant_controller_spec.js.coffee
index de350faeb2..c1d459dabf 100644
--- a/spec/javascripts/unit/darkswarm/controllers/checkout/shop_variant_controller_spec.js.coffee
+++ b/spec/javascripts/unit/darkswarm/controllers/checkout/shop_variant_controller_spec.js.coffee
@@ -27,11 +27,29 @@ describe "ShopVariantCtrl", ->
     scope.add 5
     expect(scope.variant.line_item.quantity).toEqual 6
 
+  it "adds to an invalid quantity", ->
+    scope.$apply ->
+      scope.variant.line_item.quantity = -5
+    scope.add 1
+    expect(scope.variant.line_item.quantity).toEqual 1
+
+  it "adds to an undefined quantity", ->
+    scope.$apply ->
+      scope.variant.line_item.quantity = undefined
+    scope.add 1
+    expect(scope.variant.line_item.quantity).toEqual 1
+
   it "adds to the max quantity", ->
     scope.addMax 5
     expect(scope.variant.line_item.quantity).toEqual 0
     expect(scope.variant.line_item.max_quantity).toEqual 5
 
+  it "adds to an undefined max quantity", ->
+    scope.variant.line_item.quantity = 3
+    scope.variant.line_item.max_quantity = undefined
+    scope.addMax 1
+    expect(scope.variant.line_item.max_quantity).toEqual 4
+
   it "adds to the max quantity to be at least min quantity", ->
     scope.$apply ->
       scope.variant.line_item.max_quantity = 2
@@ -74,6 +92,42 @@ describe "ShopVariantCtrl", ->
     expect(scope.canAdd(2)).toEqual true
     expect(scope.canAdd(3)).toEqual false
 
+  it "denies adding if quantity is too high", ->
+    scope.variant.on_demand = false
+    scope.variant.on_hand = 5
+    scope.variant.line_item.quantity = 7
+    scope.variant.line_item.max_quantity = 7
+
+    expect(scope.canAdd(1)).toEqual false
+    expect(scope.canAddMax(1)).toEqual false
+
+  it "allows decrease when quantity is too high", ->
+    scope.variant.on_demand = false
+    scope.variant.on_hand = 5
+    scope.variant.line_item.quantity = 7
+    scope.variant.line_item.max_quantity = 7
+    expect(scope.canAdd(-1)).toEqual true
+    expect(scope.canAddMax(-1)).toEqual true
+
+  it "allows increase when quantity is negative", ->
+    scope.variant.on_demand = false
+    scope.variant.on_hand = 5
+    scope.variant.line_item.quantity = -3
+    scope.variant.line_item.max_quantity = -3
+    expect(scope.canAdd(1)).toEqual true
+    expect(scope.canAddMax(1)).toEqual false
+
+    scope.variant.line_item.quantity = 1
+    expect(scope.canAddMax(1)).toEqual true
+
+  it "denies decrease when quantity is negative", ->
+    scope.variant.on_demand = false
+    scope.variant.on_hand = 5
+    scope.variant.line_item.quantity = -3
+    scope.variant.line_item.max_quantity = -3
+    expect(scope.canAdd(-1)).toEqual false
+    expect(scope.canAddMax(-1)).toEqual false
+
   it "denies declaring max quantity before item is in cart", ->
     expect(scope.canAddMax(1)).toEqual false