diff --git a/packages/venia-concept/src/reducers/cart.js b/packages/venia-concept/src/reducers/cart.js
index 18a0644277..5e00f4342b 100644
--- a/packages/venia-concept/src/reducers/cart.js
+++ b/packages/venia-concept/src/reducers/cart.js
@@ -3,6 +3,8 @@ import { handleActions } from 'redux-actions';
 import actions from 'src/actions/cart';
 import checkoutActions from 'src/actions/checkout';
 
+import omit from 'src/util/omit';
+
 export const name = 'cart';
 
 const initialState = {
@@ -24,7 +26,7 @@ const reducerMap = {
     },
     [actions.getDetails.receive]: (state, { payload, error }) => {
         if (error) {
-            return state;
+            return omit(state, 'guestCartId');
         }
 
         return {
diff --git a/packages/venia-concept/src/util/omit.js b/packages/venia-concept/src/util/omit.js
new file mode 100644
index 0000000000..1752c088d5
--- /dev/null
+++ b/packages/venia-concept/src/util/omit.js
@@ -0,0 +1,35 @@
+/**
+ * Return a copy of an object with one named property omitted.
+ * Short version of lodash.omit.
+ *
+ * Useful when a reducer needs to drop something from state, without using
+ * the `delete` keyword to mutate the existing state. The `delete` keyword
+ * is also a deopt.
+ *
+ * @param {object} obj -- Object to copy
+ * @param {string} omitKey -- Property name to omit
+ * @returns {object} Copy of the object with the specified key omitted.
+ *
+ * @example
+ *
+ * ```js
+ * const musketeers = {
+ *   athos: true,
+ *   porthos: true,
+ *   aramis: true
+ * };
+ *
+ * console.lot(omit(musketeers, 'athos'));
+ *
+ * {
+ *   porthos: true,
+ *   aramis: true
+ * }
+ */
+
+module.exports = (obj, omitKey) =>
+    Object.entries(obj).reduce(
+        (out, [key, value]) =>
+            key === omitKey ? out : ((out[key] = value), out),
+        {}
+    );