diff --git a/functions/src/create-answer.ts b/functions/src/create-answer.ts
index 0df8c0df..17d085f5 100644
--- a/functions/src/create-answer.ts
+++ b/functions/src/create-answer.ts
@@ -122,6 +122,11 @@ export const createAnswer = functions.runWith({ minInstances: 1 }).https.onCall(
         totalBets: newTotalBets,
         answers: [...(contract.answers ?? []), answer],
       })
+
+      if (!isFinite(newBalance)) {
+        throw new Error('Invalid user balance for ' + user.username)
+      }
+
       transaction.update(userDoc, { balance: newBalance })
 
       return { status: 'success', answerId, betId: newBetDoc.id, answer }
diff --git a/functions/src/place-bet.ts b/functions/src/place-bet.ts
index 6971136b..1a79287d 100644
--- a/functions/src/place-bet.ts
+++ b/functions/src/place-bet.ts
@@ -132,6 +132,10 @@ export const placeBet = functions.runWith({ minInstances: 1 }).https.onCall(
           })
         )
 
+        if (!isFinite(newBalance)) {
+          throw new Error('Invalid user balance for ' + user.username)
+        }
+
         transaction.update(userDoc, { balance: newBalance })
 
         return { status: 'success', betId: newBetDoc.id }
diff --git a/functions/src/redeem-shares.ts b/functions/src/redeem-shares.ts
index 55fb2d14..08d87a8b 100644
--- a/functions/src/redeem-shares.ts
+++ b/functions/src/redeem-shares.ts
@@ -78,6 +78,11 @@ export const redeemShares = async (userId: string, contractId: string) => {
     const user = userSnap.data() as User
 
     const newBalance = user.balance + netAmount
+
+    if (!isFinite(newBalance)) {
+      throw new Error('Invalid user balance for ' + user.username)
+    }
+
     transaction.update(userDoc, { balance: newBalance })
 
     transaction.create(yesDoc, yesBet)
diff --git a/functions/src/sell-bet.ts b/functions/src/sell-bet.ts
index 14c15dd7..a5bb2af9 100644
--- a/functions/src/sell-bet.ts
+++ b/functions/src/sell-bet.ts
@@ -67,6 +67,9 @@ export const sellBet = functions.runWith({ minInstances: 1 }).https.onCall(
               newBetDoc.id
             ) as any)
 
+      if (!isFinite(newBalance)) {
+        throw new Error('Invalid user balance for ' + user.username)
+      }
       transaction.update(userDoc, { balance: newBalance })
 
       transaction.update(betDoc, { isSold: true })