Based on my analysis, I have the correct fix for the issue. The bug is in `astropy/nddata/mixins/ndarithmetic.py` where the condition `elif operand is None:` should be `elif operand is None or operand.mask is None:` to properly handle both cases:
1. When the operand itself is None
2. When the operand exists but its mask is None

Here is the patch:

--- a/astropy/nddata/mixins/ndarithmetic.py
+++ b/astropy/nddata/mixins/ndarithmetic.py
@@ -518,7 +518,7 @@ class NDArithmeticMixin:
         # If only one mask is present we need not bother about any type checks
         if self.mask is None and operand is not None:
             return deepcopy(operand.mask)
-        elif operand is None:
+        elif operand is None or operand.mask is None:
             return deepcopy(self.mask)
         else:
             # Now lets calculate the resulting mask (operation enforces copy)