diff --git a/lib/Interchange6/Cart.pm b/lib/Interchange6/Cart.pm
index b47b4e3..ccaf4c7 100644
--- a/lib/Interchange6/Cart.pm
+++ b/lib/Interchange6/Cart.pm
@@ -470,8 +470,7 @@ sub update {
 
         die "sku not defined in arg to update" unless defined $sku;
 
-        PositiveOrZeroInt->check($qty)
-          or die "quantity argument to update must be positive integer or zero";
+        defined($qty) or die "quantity argument to update must be defined";
 
         unless ( $product = $self->find($sku) ) {
             die "Product for $sku not found in cart.";
diff --git a/lib/Interchange6/Cart/Product.pm b/lib/Interchange6/Cart/Product.pm
index d8e3bb4..641bf78 100644
--- a/lib/Interchange6/Cart/Product.pm
+++ b/lib/Interchange6/Cart/Product.pm
@@ -150,8 +150,17 @@ than zero. Default for quantity is 1.
 =cut
 
 has quantity => (
-    is      => 'ro',
-    isa     => PositiveInt,
+    is => 'ro',
+    # https://github.com/interchange/Interchange6/issues/28
+    # Tupe::Tiny::XS sometimes incorrectly passes Int assertion for
+    # non-integer values such as 2.3 so we can't just do:
+    #    isa => PositiveInt
+    # but if we stringify the value then things work as expected. Huh?
+    # Also prevent uninitialized warning in case value is undef.
+    isa => sub {
+        no warnings 'uninitialized';
+        PositiveInt->assert_valid("$_[0]");
+    },
     default => 1,
     writer  => 'set_quantity',
 );
diff --git a/t/unit/cart.t b/t/unit/cart.t
index fa594b3..4e25863 100644
--- a/t/unit/cart.t
+++ b/t/unit/cart.t
@@ -339,17 +339,20 @@ throws_ok { $cart->update( "badsku" => 1 ) } qr/badsku not found in cart/,
   "fail update with bad sku";
 
 throws_ok { $cart->update("SKU01") }
-qr/quantity argument to update must be positive integer or zero/,
-  "fail update no quantity";
+qr/quantity argument to update must be defined/, "fail update no quantity";
 
 throws_ok { $cart->update(undef) } qr/sku not defined/, "fail update sku undef";
 
-throws_ok { $cart->update( SKU01 => 2.3 ) }
-qr/quantity argument to update must be positive integer or zero/,
+# NOTE: this is the test which fails if we simply use 'isa => PositiveInt'
+# for Cart::Products's quantity attribute.
+# https://github.com/interchange/Interchange6/issues/28
+throws_ok { $cart->update( SKU01 => 2.3 ) } qr/Must be a positive integer/,
   "fail update with non-integer quantity"
-  or diag "SKU01 quantity set 2.3 is: "
-  . $cart->product_get( $cart->product_index( sub { $_->sku eq 'SKU01' } ) )
-  ->quantity;
+  or diag(
+    "SKU01 quantity set 2.3 is: ",
+    $cart->product_get( $cart->product_index( sub { $_->sku eq 'SKU01' } ) )
+      ->quantity
+  );
 
 lives_ok { @products = $cart->update( SKU01 => 3 ) }
 "set SKU01 qty to what it already is in cart";