diff --git a/src/main/java/swiss/qpq/gajumaru/core/tools/Grids.java b/src/main/java/swiss/qpq/gajumaru/core/tools/Grids.java
index dfe0298..5c46ca3 100644
--- a/src/main/java/swiss/qpq/gajumaru/core/tools/Grids.java
+++ b/src/main/java/swiss/qpq/gajumaru/core/tools/Grids.java
@@ -93,13 +93,15 @@ public final class Grids {
             if (path.startsWith("/1/s/")) {
                 String recipient = path.substring(5);
                 Map<String, String> qargs = parseQuery(query);
-                BigInteger amount = new BigInteger(qargs.getOrDefault("a", "0"));
+                String a = qargs.get("a");
+                BigInteger amount = (a == null || a.isEmpty()) ? BigInteger.ZERO : new BigInteger(a);
                 byte[] payload = decodePayload(qargs.getOrDefault("p", ""));
                 return ParseResult.spend(Context.CHAIN, host, recipient, amount, payload);
             } else if (path.startsWith("/1/t/")) {
                 String recipient = path.substring(5);
                 Map<String, String> qargs = parseQuery(query);
-                BigInteger amount = new BigInteger(qargs.getOrDefault("a", "0"));
+                String a = qargs.get("a");
+                BigInteger amount = (a == null || a.isEmpty()) ? BigInteger.ZERO : new BigInteger(a);
                 byte[] payload = decodePayload(qargs.getOrDefault("p", ""));
                 String location = host + (uri.getPort() != -1 ? ":" + uri.getPort() : "");
                 return ParseResult.transfer(Context.NODE, location, recipient, amount, payload);