diff --git a/lib/ip.js b/lib/ip.js
index c1799a8..ab010bd 100644
--- a/lib/ip.js
+++ b/lib/ip.js
@@ -400,6 +400,9 @@ ip.address = function(name, family) {
 };
 
 ip.toLong = function(ip) {
+  if (!this.isV4Format(ip)) {
+    throw new Error('.toLong requires valid IPv4 as input: ' + ip);
+  }
   var ipl = 0;
   ip.split('.').forEach(function(octet) {
     ipl <<= 8;
diff --git a/test/api-test.js b/test/api-test.js
index 2e390f9..b39fcfa 100644
--- a/test/api-test.js
+++ b/test/api-test.js
@@ -386,7 +386,7 @@ describe('IP library for node.js', function() {
             var addr = ip.address(nic, 'ipv6');
             assert.ok(!addr || net.isIPv6(addr));
           });
-        })
+        });
       });
     });
   });
@@ -396,6 +396,17 @@ describe('IP library for node.js', function() {
       assert.equal(ip.toLong('127.0.0.1'), 2130706433);
       assert.equal(ip.toLong('255.255.255.255'), 4294967295);
     });
+    it('should be able handle bad input', function() {
+      assert.throws(function () {
+        ip.toLong('fd12:3456:789a:1::1');
+      }, Error);
+      assert.throws(function () {
+        ip.toLong(null);
+      }, Error);
+      assert.throws(function () {
+        ip.toLong(undefined);
+      }, Error);
+    });
   });
 
   describe('fromLong() method', function() {