diff --git a/index.js b/index.js
index a1d2ea4..8b3455a 100644
--- a/index.js
+++ b/index.js
@@ -28,7 +28,7 @@ module.exports = function(val, options) {
   var type = typeof val;
   if (type === 'string' && val.length > 0) {
     return parse(val);
-  } else if (type === 'number' && isNaN(val) === false) {
+  } else if (type === 'number' && isFinite(val)) {
     return options.long ? fmtLong(val) : fmtShort(val);
   }
   throw new Error(
diff --git a/tests.js b/tests.js
index 9427390..6a44931 100644
--- a/tests.js
+++ b/tests.js
@@ -283,4 +283,16 @@ describe('ms(invalid inputs)', function() {
       ms(NaN);
     }).to.throwError();
   });
+
+  it('should throw an error, when ms(Infinity)', function() {
+    expect(function() {
+      ms(Infinity);
+    }).to.throwError();
+  });
+
+  it('should throw an error, when ms(-Infinity)', function() {
+    expect(function() {
+      ms(-Infinity);
+    }).to.throwError();
+  });
 });