diff --git a/flag_encoder/index.html b/flag_encoder/index.html
index 1070c07..025b272 100644
--- a/flag_encoder/index.html
+++ b/flag_encoder/index.html
@@ -45,7 +45,7 @@
 			} else {
 				outpObscuredFlag.value = addressFromKey(k);
 				var m = inpContestantAddress.value;
-				if (m.slice(0, 2) !== "0x") {
+				if (!/^0x[0-9a-fA-F]{40}$/.test(m)) {
 					outpSignature.value = "";
 				} else {
 					outpSignature.value = signMessage(m, k);