diff --git a/app/scripts/lib/nonce-tracker.js b/app/scripts/lib/nonce-tracker.js
index e0e065d82..08f1e1e86 100644
--- a/app/scripts/lib/nonce-tracker.js
+++ b/app/scripts/lib/nonce-tracker.js
@@ -115,13 +115,21 @@ class NonceTracker {
   }
 
   _getHighestNonce (txList) {
-    const nonces = txList.map((txMeta) => parseInt(txMeta.txParams.nonce, 16))
+    const nonces = txList.map((txMeta) => {
+      const nonce = txMeta.txParams.nonce
+      assert(typeof nonce, 'string', 'nonces should be hex strings')
+      return parseInt(nonce, 16)
+    })
     const highestNonce = Math.max.apply(null, nonces)
     return highestNonce
   }
 
   _getHighestContinuousFrom (txList, startPoint) {
-    const nonces = txList.map((txMeta) => parseInt(txMeta.txParams.nonce, 16))
+    const nonces = txList.map((txMeta) => {
+      const nonce = txMeta.txParams.nonce
+      assert(typeof nonce, 'string', 'nonces should be hex strings')
+      return parseInt(nonce, 16)
+    })
 
     let highest = startPoint
     while (nonces.includes(highest)) {