diff --git a/index.js b/index.js
index a1d2ea4..a2d1969 100644
--- a/index.js
+++ b/index.js
@@ -50,7 +50,7 @@ function parse(str) {
   if (str.length > 100) {
     return;
   }
-  var match = /^((?:\d+)?-?(?:\d+)?\.?\d+) *(milliseconds?|msecs?|ms|seconds?|secs?|s|minutes?|mins?|m|hours?|hrs?|h|days?|d|weeks?|w|years?|yrs?|y)?$/i.exec(
+  var match = /^(-?(?:\d+)?\.?\d+) *(milliseconds?|msecs?|ms|seconds?|secs?|s|minutes?|mins?|m|hours?|hrs?|h|days?|d|weeks?|w|years?|yrs?|y)?$/i.exec(
     str
   );
   if (!match) {
diff --git a/tests.js b/tests.js
index 9427390..9141689 100644
--- a/tests.js
+++ b/tests.js
@@ -55,6 +55,7 @@ describe('ms(string)', function() {
 
   it('should return NaN if invalid', function() {
     expect(isNaN(ms('☃'))).to.be(true);
+    expect(isNaN(ms('10-.5'))).to.be(true);
   });
 
   it('should be case-insensitive', function() {