diff --git a/sign.js b/sign.js
index c608f4d..de80b02 100644
--- a/sign.js
+++ b/sign.js
@@ -9,8 +9,8 @@ var isString = require('lodash.isstring');
 var once = require('lodash.once');
 
 var sign_options_schema = {
-  expiresIn: { isValid: function(value) { return isInteger(value) || isString(value); }, message: '"expiresIn" should be a number of seconds or string representing a timespan' },
-  notBefore: { isValid: function(value) { return isInteger(value) || isString(value); }, message: '"notBefore" should be a number of seconds or string representing a timespan' },
+  expiresIn: { isValid: function(value) { return isInteger(value) || (isString(value) && value); }, message: '"expiresIn" should be a number of seconds or string representing a timespan' },
+  notBefore: { isValid: function(value) { return isInteger(value) || (isString(value) && value); }, message: '"notBefore" should be a number of seconds or string representing a timespan' },
   audience: { isValid: function(value) { return isString(value) || Array.isArray(value); }, message: '"audience" must be a string or array' },
   algorithm: { isValid: includes.bind(null, ['RS256', 'RS384', 'RS512', 'ES256', 'ES384', 'ES512', 'HS256', 'HS384', 'HS512', 'none']), message: '"algorithm" must be a valid string enum value' },
   header: { isValid: isPlainObject, message: '"header" must be an object' },
diff --git a/test/claim-exp.test.js b/test/claim-exp.test.js
index 816d12e..dfe248b 100644
--- a/test/claim-exp.test.js
+++ b/test/claim-exp.test.js
@@ -46,12 +46,12 @@ describe('expires', function() {
       });
     });
 
-    // TODO this should throw the same error as other invalid inputs
     it(`should error with with value ''`, function (done) {
       signWithExpiresIn('', {}, (err) => {
         testUtils.asyncCheck(done, () => {
           expect(err).to.be.instanceOf(Error);
-          expect(err).to.have.property('message', 'val is not a non-empty string or a valid number. val=""');
+          expect(err).to.have.property('message')
+            .match(/"expiresIn" should be a number of seconds or string representing a timespan/);
         });
       });
     });
diff --git a/test/claim-nbf.test.js b/test/claim-nbf.test.js
index f36396c..8290705 100644
--- a/test/claim-nbf.test.js
+++ b/test/claim-nbf.test.js
@@ -51,7 +51,8 @@ describe('not before', function() {
       signWithNotBefore('', {}, (err) => {
         testUtils.asyncCheck(done, () => {
           expect(err).to.be.instanceOf(Error);
-          expect(err).to.have.property('message', 'val is not a non-empty string or a valid number. val=""');
+          expect(err).to.have.property('message')
+            .match(/"notBefore" should be a number of seconds or string representing a timespan/);
         });
       });
     });