diff --git a/.changeset/rich-geese-complain.md b/.changeset/rich-geese-complain.md
new file mode 100644
index 00000000000..f60df47d644
--- /dev/null
+++ b/.changeset/rich-geese-complain.md
@@ -0,0 +1,5 @@
+---
+"@apollo/client": patch
+---
+
+Fix error "Cannot convert object to primitive value"
diff --git a/src/cache/inmemory/writeToStore.ts b/src/cache/inmemory/writeToStore.ts
index a88c875e268..9e7daeb7ebe 100644
--- a/src/cache/inmemory/writeToStore.ts
+++ b/src/cache/inmemory/writeToStore.ts
@@ -873,7 +873,7 @@ For more information about these options, please refer to the documentation:
         " have an ID or a custom merge function, or "
     : "",
     typeDotName,
-    existing,
-    incoming
+    { ...existing },
+    { ...incoming }
   );
 }