diff --git a/onchain/ProvethVerifier.sol b/onchain/ProvethVerifier.sol
index 5af77db..e75ee18 100644
--- a/onchain/ProvethVerifier.sol
+++ b/onchain/ProvethVerifier.sol
@@ -406,6 +406,12 @@ contract ProvethVerifier {
                     } else {
                         nodeHashHash = keccak256(node[nibble].toRlpBytes());
                     }
+
+                    // Sanity
+                    if (i == stack.length - 1) {
+                        // need to process the child now
+                        revert();
+                    }
                 } else {
                     // we have consumed the entire mptKey, so we need to look at what's contained in this node.
 
@@ -419,5 +425,8 @@ contract ProvethVerifier {
                 }
             }
         }
+
+        // If execution reaches here, the proof was inconclusive or malformed
+        revert("Invalid MPT proof");
     }
 }