diff --git a/foundry/src/TychoRouter.sol b/foundry/src/TychoRouter.sol
index b9f4d1c..84d686d 100644
--- a/foundry/src/TychoRouter.sol
+++ b/foundry/src/TychoRouter.sol
@@ -18,6 +18,7 @@ import {LibSwap} from "../lib/LibSwap.sol";
 error TychoRouter__WithdrawalFailed();
 error TychoRouter__AddressZero();
 error TychoRouter__NegativeSlippage(uint256 amount, uint256 minAmount);
+error TychoRouter__AmountInNotFullySpent(uint256 leftoverAmount);
 error TychoRouter__MessageValueMismatch(uint256 value, uint256 amount);
 
 contract TychoRouter is
@@ -153,6 +154,7 @@ contract TychoRouter is
         // For native ETH, assume funds already in our router. Else, transfer and handle approval.
         if (wrapEth) {
             _wrapETH(amountIn);
+            tokenIn = address(_weth);
         } else if (tokenIn != address(0)) {
             permit2.permit(msg.sender, permitSingle, signature);
             permit2.transferFrom(
@@ -175,6 +177,11 @@ contract TychoRouter is
             revert TychoRouter__NegativeSlippage(amountOut, minAmountOut);
         }
 
+        uint256 leftoverAmountIn = IERC20(tokenIn).balanceOf(address(this));
+        if (leftoverAmountIn > 0) {
+            revert TychoRouter__AmountInNotFullySpent(leftoverAmountIn);
+        }
+
         if (unwrapEth) {
             _unwrapETH(amountOut);
         }
diff --git a/foundry/test/TychoRouter.t.sol b/foundry/test/TychoRouter.t.sol
index a842595..af6d925 100644
--- a/foundry/test/TychoRouter.t.sol
+++ b/foundry/test/TychoRouter.t.sol
@@ -733,4 +733,56 @@ contract TychoRouterTest is TychoRouterTestSetup {
         // all of it (and thus our splits are correct).
         assertEq(IERC20(WETH_ADDR).balanceOf(tychoRouterAddr), 0);
     }
+
+    function testSwapAmountInNotFullySpent() public {
+        // Trade 1 WETH for DAI with 1 swap on Uniswap V2
+        // Has invalid data as input! There is only one swap with 60% of the input amount
+        uint256 amountIn = 1 ether;
+        deal(WETH_ADDR, ALICE, amountIn);
+
+        vm.startPrank(ALICE);
+
+        (
+            IAllowanceTransfer.PermitSingle memory permitSingle,
+            bytes memory signature
+        ) = handlePermit2Approval(WETH_ADDR, amountIn);
+
+        bytes memory protocolData = encodeUniswapV2Swap(
+            WETH_ADDR, WETH_DAI_POOL, tychoRouterAddr, false
+        );
+
+        bytes memory swap = encodeSwap(
+            uint8(0),
+            uint8(1),
+            (0xffffff * 60) / 100, // 60%
+            address(usv2Executor),
+            bytes4(0),
+            protocolData
+        );
+
+        bytes[] memory swaps = new bytes[](1);
+        swaps[0] = swap;
+
+        vm.expectRevert(
+            abi.encodeWithSelector(
+                TychoRouter__AmountInNotFullySpent.selector, 400000000000000000
+            )
+        );
+
+        tychoRouter.swap(
+            amountIn,
+            WETH_ADDR,
+            DAI_ADDR,
+            0,
+            false,
+            false,
+            2,
+            ALICE,
+            permitSingle,
+            signature,
+            pleEncode(swaps)
+        );
+
+        vm.stopPrank();
+    }
 }