diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index c6828e3..f904696 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -3174,6 +3174,31 @@
     END
     v9.CheckSourceSuccess(lines)
   endif
+
+  lines =<< trim END
+    vim9script
+    class A
+    endclass
+
+    def F()
+      A += 3
+    enddef
+    F()
+  END
+  v9.CheckScriptFailure(lines, 'E1405: Class "A" cannot be used as a value')
+
+  lines =<< trim END
+    vim9script
+    class A
+    endclass
+
+    var o = A.new()
+    def F()
+      o += 4
+    enddef
+    F()
+  END
+  v9.CheckScriptFailure(lines, 'E1411: Missing dot after object "o"')
 enddef
 
 " Test for checking the argument type of a def function