check for negatives and zero in testTarget.

lib/bcoin/miner.js

@@ -526,7 +526,7 @@ MinerBlock.prototype.findNonce = function findNonce() {
   // The heart and soul of the miner: match the target.
   while (block.nonce <= 0xffffffff) {
     // Hash and test against the next target
-    if (rcmp(utils.dsha256(data), target) < 0)
+    if (rcmp(utils.dsha256(data), target) <= 0)
       return true;
 
     // Increment the nonce to get a different hash

lib/bcoin/utils.js

@@ -1062,7 +1062,15 @@ utils.testTarget = function testTarget(hash, target) {
   if (typeof target === 'number')
     target = utils.fromCompact(target);
 
-  return new bn(hash, 'le').cmp(target) < 0;
+  if (target.isNeg() || target.cmpn(0) === 0)
+    return false;
+
+  hash = new bn(hash, 'le');
+
+  if (hash.cmp(target) > 0)
+    return false;
+
+  return true;
 };
 
 /**