diff --git a/lib/primitives/mtx.js b/lib/primitives/mtx.js
index 1a8b88e8..b7459002 100644
--- a/lib/primitives/mtx.js
+++ b/lib/primitives/mtx.js
@@ -1508,7 +1508,7 @@ CoinSelector.prototype.fromOptions = function fromOptions(options) {
 
   if (options.subtractFee != null) {
     if (typeof options.subtractFee === 'number') {
-      assert(util.isNumber(options.subtractFee));
+      assert(util.isUInt32(options.subtractFee));
       this.subtractFee = options.subtractFee;
       this.shouldSubtract = true;
     } else {
diff --git a/lib/primitives/tx.js b/lib/primitives/tx.js
index 606f781b..8a7f9791 100644
--- a/lib/primitives/tx.js
+++ b/lib/primitives/tx.js
@@ -2278,14 +2278,13 @@ TX.prototype.fromJSON = function fromJSON(json) {
   var i, input, output;
 
   assert(json, 'TX data is required.');
-  assert(util.isUInt32(json.version));
-  assert(util.isUInt8(json.flag));
-  assert(Array.isArray(json.inputs));
-  assert(Array.isArray(json.outputs));
-  assert(util.isUInt32(json.locktime));
+  assert(util.isUInt32(json.version), 'Version must be a uint32.');
+  assert(util.isUInt8(json.flag), 'Flag must be a uint8.');
+  assert(Array.isArray(json.inputs), 'Inputs must be an array.');
+  assert(Array.isArray(json.outputs), 'Outputs must be an array.');
+  assert(util.isUInt32(json.locktime), 'Locktime must be a uint32.');
 
   this.version = json.version;
-
   this.flag = json.flag;
 
   for (i = 0; i < json.inputs.length; i++) {