diff --git a/crates/tools/src/campaign_totals.rs b/crates/tools/src/campaign_totals.rs
index 5e7f95e..25800b6 100644
--- a/crates/tools/src/campaign_totals.rs
+++ b/crates/tools/src/campaign_totals.rs
@@ -1,3 +1,4 @@
+use anyhow::{anyhow, Result};
 use std::collections::HashMap;
 
 /// In-memory store for per-campaign donation totals, grouped by asset.
@@ -16,12 +17,16 @@ impl CampaignTotals {
     }
 
     /// Adds `amount` to the running total for `campaign_id` + `asset` and returns the new total.
-    #[must_use]
+    ///
+    /// Returns `Err` if `amount` is non-positive or if the addition would overflow.
     #[inline]
-    pub fn increment(&mut self, campaign_id: u64, asset: &str, amount: i128) -> i128 {
+    pub fn increment(&mut self, campaign_id: u64, asset: &str, amount: i128) -> Result<i128> {
+        if amount <= 0 {
+            return Err(anyhow!("CampaignTotals::increment requires positive amount; got {}", amount));
+        }
         let entry = self.asset_totals.entry((campaign_id, asset.to_string())).or_insert(0);
-        *entry += amount;
-        *entry
+        *entry = entry.checked_add(amount).ok_or_else(|| anyhow!("overflow in CampaignTotals"))?;
+        Ok(*entry)
     }
 
     /// Returns the total for a specific `campaign_id` + `asset`, or 0 if none recorded.
@@ -64,16 +69,16 @@ mod tests {
     #[test]
     fn increments_per_asset() {
         let mut totals = CampaignTotals::new();
-        totals.increment(1, "XLM", 500);
-        totals.increment(1, "XLM", 300);
+        totals.increment(1, "XLM", 500).unwrap();
+        totals.increment(1, "XLM", 300).unwrap();
         assert_eq!(totals.get(1, "XLM"), 800);
     }
 
     #[test]
     fn different_assets_are_independent() {
         let mut totals = CampaignTotals::new();
-        totals.increment(1, "XLM", 100);
-        totals.increment(1, "USDC", 200);
+        totals.increment(1, "XLM", 100).unwrap();
+        totals.increment(1, "USDC", 200).unwrap();
         assert_eq!(totals.get(1, "XLM"), 100);
         assert_eq!(totals.get(1, "USDC"), 200);
     }
@@ -81,8 +86,8 @@ mod tests {
     #[test]
     fn different_campaigns_are_independent() {
         let mut totals = CampaignTotals::new();
-        totals.increment(1, "XLM", 100);
-        totals.increment(2, "XLM", 200);
+        totals.increment(1, "XLM", 100).unwrap();
+        totals.increment(2, "XLM", 200).unwrap();
         assert_eq!(totals.get(1, "XLM"), 100);
         assert_eq!(totals.get(2, "XLM"), 200);
     }
@@ -90,8 +95,8 @@ mod tests {
     #[test]
     fn get_all_assets_returns_correct_map() {
         let mut totals = CampaignTotals::new();
-        totals.increment(1, "XLM", 500);
-        totals.increment(1, "USDC", 300);
+        totals.increment(1, "XLM", 500).unwrap();
+        totals.increment(1, "USDC", 300).unwrap();
         let map = totals.get_all_assets(1);
         assert_eq!(map.get("XLM"), Some(&500));
         assert_eq!(map.get("USDC"), Some(&300));
@@ -100,8 +105,30 @@ mod tests {
     #[test]
     fn campaign_total_aggregates_all_assets() {
         let mut totals = CampaignTotals::new();
-        totals.increment(1, "XLM", 500);
-        totals.increment(1, "USDC", 300);
+        totals.increment(1, "XLM", 500).unwrap();
+        totals.increment(1, "USDC", 300).unwrap();
         assert_eq!(totals.get_campaign_total(1), 800);
     }
+
+    #[test]
+    fn rejects_negative_amount() {
+        let mut totals = CampaignTotals::new();
+        let err = totals.increment(1, "XLM", -100).unwrap_err();
+        assert!(err.to_string().contains("positive amount"));
+    }
+
+    #[test]
+    fn rejects_zero_amount() {
+        let mut totals = CampaignTotals::new();
+        let err = totals.increment(1, "XLM", 0).unwrap_err();
+        assert!(err.to_string().contains("positive amount"));
+    }
+
+    #[test]
+    fn rejects_overflow() {
+        let mut totals = CampaignTotals::new();
+        totals.increment(1, "XLM", i128::MAX).unwrap();
+        let err = totals.increment(1, "XLM", 1).unwrap_err();
+        assert!(err.to_string().contains("overflow"));
+    }
 }