fix

cashu/mint/ledger.py

@@ -668,6 +668,10 @@ async def melt_quote(
         # and therefore respond with internal transaction fees (0 for now)
         mint_quote = await self.crud.get_mint_quote(request=request, db=self.db)
         if mint_quote and mint_quote.unit == melt_quote.unit:
+            # check if the melt quote is partial and error if it is.
+            # it's just not possible to handle this case
+            if melt_quote.is_mpp:
+                raise TransactionError("multi-part internal payments are not possible. try with a regular payment.")
             payment_quote = self.create_internal_melt_quote(mint_quote, melt_quote)
         else:
             # not internal

tests/test_wallet_regtest_mpp.py

@@ -11,6 +11,7 @@
 from cashu.wallet.wallet import Wallet
 from tests.conftest import SERVER_ENDPOINT
 from tests.helpers import (
+    assert_err,
     get_real_invoice,
     is_fake,
     partial_pay_real_invoice,
@@ -139,3 +140,15 @@ async def pay_mpp(amount: int, proofs: List[Proof], delay: float = 0.0):
     await asyncio.sleep(2)
 
     assert wallet.balance <= 384 - 64
+
+@pytest.mark.asyncio
+@pytest.mark.skipif(is_fake, reason="only regtest")
+async def test_regtest_internal_mpp_melt_quotes(wallet: Wallet, ledger: Ledger):
+    # make sure wallet knows the backend supports mpp
+    assert wallet.mint_info.supports_mpp("bolt11", wallet.unit)
+
+    # create a mint quote
+    mint_quote = wallet.request_mint(128)
+
+    # try and create a multi-part melt quote
+    assert_err(wallet.melt_quote(mint_quote.request, 100), "multi-part internal payments are not possible. try with a regular payment.")