diff --git a/electrum/gui/qt/transaction_dialog.py b/electrum/gui/qt/transaction_dialog.py
index 1c05185c8..bf0ef39d8 100644
--- a/electrum/gui/qt/transaction_dialog.py
+++ b/electrum/gui/qt/transaction_dialog.py
@@ -905,7 +905,7 @@ class TxDialog(QDialog, MessageBoxMixin):
             fee_rate = Decimal(fee) / size  # sat/byte
             fee_str += '  ( %s ) ' % self.main_window.format_fee_rate(fee_rate * 1000)
             if isinstance(self.tx, PartialTransaction):
-                invoice_amt = amount
+                invoice_amt = abs(amount)
                 fee_warning_tuple = self.wallet.get_tx_fee_warning(
                     invoice_amt=invoice_amt, tx_size=size, fee=fee)
                 if fee_warning_tuple:
diff --git a/electrum/wallet.py b/electrum/wallet.py
index 8f24fbf43..ed146b7b2 100644
--- a/electrum/wallet.py
+++ b/electrum/wallet.py
@@ -3178,6 +3178,8 @@ class Abstract_Wallet(ABC, Logger, EventListener):
             tx_size: int,
             fee: int) -> Optional[Tuple[bool, str, str]]:
 
+        assert invoice_amt >= 0, f"{invoice_amt=!r} must be non-negative satoshis"
+        assert fee >= 0, f"{fee=!r} must be non-negative satoshis"
         feerate = Decimal(fee) / tx_size  # sat/byte
         fee_ratio = Decimal(fee) / invoice_amt if invoice_amt else 0
         long_warning = None