diff --git a/lib/address.js b/lib/address.js
index 5d73e72..18d0d4b 100644
--- a/lib/address.js
+++ b/lib/address.js
@@ -7,9 +7,9 @@
 "use strict";
 
 require("object.assign").shim();
-var assert = require("assert");
 var bufferEqual = require("buffer-equal");
 var bs58 = require("bs58");
+var assert = require("./util").assert;
 var var_int = require("./structs").var_int;
 var bmcrypto = require("./crypto");
 
diff --git a/lib/structs.js b/lib/structs.js
index ae13c98..e01a74f 100644
--- a/lib/structs.js
+++ b/lib/structs.js
@@ -7,8 +7,8 @@
 "use strict";
 
 require("object.assign").shim();
-var assert = require("assert");
 var bufferEqual = require("buffer-equal");
+var assert = require("./util").assert;
 var bmcrypto = require("./crypto");
 
 function isAscii(str) {
diff --git a/lib/util.js b/lib/util.js
new file mode 100644
index 0000000..ffe85a2
--- /dev/null
+++ b/lib/util.js
@@ -0,0 +1,7 @@
+"use strict";
+
+exports.assert = function(condition, message) {
+  if (!condition) {
+    throw new Error(message || "Assertion failed");
+  }
+};
diff --git a/lib/wif.js b/lib/wif.js
index ae5041b..b721bf3 100644
--- a/lib/wif.js
+++ b/lib/wif.js
@@ -6,9 +6,9 @@
 
 "use strict";
 
-var assert = require("assert");
 var bufferEqual = require("buffer-equal");
 var bs58 = require("bs58");
+var assert = require("./util").assert;
 var bmcrypto = require("./crypto");
 
 // Compute the WIF checksum for the given data.