Can't craft spell damage on wand with open prefix slot

I got a rare wand that doesn't have a spell damage prefix and it has 1 free prefix and 1 free suffix but I can't craft spell damage on it.
It has lightning damage and I can craft more lightning damage on it, so I think this is a bug...


Rarity: Rare
Cataclysm Thirst
Imbued Wand
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Wand
Physical Damage: 24-44
Critical Strike Chance: 7.00%
Attacks per Second: 1.50
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Requirements:
Level: 60
Int: 188
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Sockets: B-B
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Item Level: 85
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33% increased Spell Damage
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27% increased Lightning Damage
Adds 4 to 74 Lightning Damage to Spells
105% increased Critical Strike Chance for Spells
+283 to Accuracy Rating

Not a bug; Increased Spell Damage and Increased (Element) Damage are mutually exclusive.

Ah now I understand it a bit better . The lightning damage mod is a prefix (which you excluded from your explanation) and that one is mutually exclusive with increased spell damage (also prefix). The increased lightning damage I can craft is a suffix...

Then I agree it's not a bug :(