diff --git a/src/PostageStamp.sol b/src/PostageStamp.sol
index 00cdaa3f..23e4114e 100644
--- a/src/PostageStamp.sol
+++ b/src/PostageStamp.sol
@@ -566,7 +566,8 @@ contract PostageStamp is AccessControl, Pausable {
     }
 
     function minimumInitialBalancePerChunk() public view returns (uint256) {
-        return minimumValidityBlocks * lastPrice;
+        // Cast to uint256 before multiplying to avoid uint64 overflow.
+        return uint256(minimumValidityBlocks) * uint256(lastPrice);
     }
 
     /**