This is working as intended. You can only suspend a card if you could begin to cast that spell.
Comprehensive Rules 702.62a. Suspend is a keyword that represents three abilities. The first is a static ability that functions while the card with suspend is in a player's hand. The second and third are triggered abilities that function in the exile zone. "Suspend N--[cost" means "If you could begin to cast this card by putting it onto the stack from your hand, you may pay [cost] and exile it with N time counters on it. This action doesn't use the stack," and "At the beginning of your upkeep, if this card is suspended, remove a time counter from it," and "When the last time counter is removed from this card, if it's exiled, you may play it without paying its mana cost if able. If you don't, it remains exiled. If you cast a creature spell this way, it gains haste until you lose control of the spell or the permanent it becomes."
