diff --git a/Tests/test_imagemath.py b/Tests/test_imagemath.py
index 22de86c7c..9a0326ece 100644
--- a/Tests/test_imagemath.py
+++ b/Tests/test_imagemath.py
@@ -64,6 +64,11 @@ def test_prevent_exec(expression):
         ImageMath.eval(expression)
 
 
+def test_prevent_double_underscores():
+    with pytest.raises(ValueError):
+        ImageMath.eval("1", {"__": None})
+
+
 def test_logical():
     assert pixel(ImageMath.eval("not A", images)) == 0
     assert pixel(ImageMath.eval("A and B", images)) == "L 2"
diff --git a/src/PIL/ImageMath.py b/src/PIL/ImageMath.py
index cf108e258..fd7d78d45 100644
--- a/src/PIL/ImageMath.py
+++ b/src/PIL/ImageMath.py
@@ -234,13 +234,14 @@ def eval(expression, _dict={}, **kw):
 
     # build execution namespace
     args = ops.copy()
-    args.update(_dict)
-    args.update(kw)
-    for k, v in args.items():
-        if '__' in k or hasattr(__builtins__, k):
+    for k in list(_dict.keys()) + list(kw.keys()):
+        if "__" in k or hasattr(__builtins__, k):
             msg = f"'{k}' not allowed"
             raise ValueError(msg)
 
+    args.update(_dict)
+    args.update(kw)
+    for k, v in args.items():
         if hasattr(v, "im"):
             args[k] = _Operand(v)