diff --git a/src/convert-expression/step-values-conversion.js b/src/convert-expression/step-values-conversion.js
index e85218a..01b13ab 100644
--- a/src/convert-expression/step-values-conversion.js
+++ b/src/convert-expression/step-values-conversion.js
@@ -2,16 +2,20 @@
 
 module.exports = (() => {
     function convertSteps(expressions){
-        const stepValuePattern = /^(.+)\/(\d+)$/;
-        for(let i = 0; i < expressions.length; i++){
-            const match = stepValuePattern.exec(expressions[i]);
-            const isStepValue = match !== null && match.length > 0;
+        var stepValuePattern = /^(.+)\/(\w+)$/;
+        for(var i = 0; i < expressions.length; i++){
+            var match = stepValuePattern.exec(expressions[i]);
+            var isStepValue = match !== null && match.length > 0;
             if(isStepValue){
-                const values = match[1].split(',');
-                const stepValues = [];
-                const divider = parseInt(match[2], 10);
-                for(let j = 0; j <= values.length; j++){
-                    const value = parseInt(values[j], 10);
+                var baseDivider = match[2];
+                if(isNaN(baseDivider)){
+                    throw baseDivider + ' is not a valid step value';
+                }
+                var values = match[1].split(',');
+                var stepValues = [];
+                var divider = parseInt(baseDivider, 10);
+                for(var j = 0; j <= values.length; j++){
+                    var value = parseInt(values[j], 10);
                     if(value % divider === 0){
                         stepValues.push(value);
                     }
diff --git a/test/convert-expression/step-values-conversion-test.js b/test/convert-expression/step-values-conversion-test.js
index dde2377..9221d96 100644
--- a/test/convert-expression/step-values-conversion-test.js
+++ b/test/convert-expression/step-values-conversion-test.js
@@ -4,10 +4,16 @@ const { expect } = require('chai');
 const conversion = require('../../src/convert-expression/step-values-conversion');
 
 describe('step-values-conversion.js', () => {
-    it('shuld convert step values', () => {
-        let expressions = '1,2,3,4,5,6,7,8,9,10/2 0,1,2,3,4,5,6,7,8,9/5 * * * *'.split(' ');
+    it('should convert step values', () => {
+        var expressions = '1,2,3,4,5,6,7,8,9,10/2 0,1,2,3,4,5,6,7,8,9/5 * * * *'.split(' ');
         expressions = conversion(expressions);
         expect(expressions[0]).to.equal('2,4,6,8,10');
         expect(expressions[1]).to.equal('0,5');
     });
+
+    it('should throw an error if step value is not a number', () => {
+        var expressions = '1,2,3,4,5,6,7,8,9,10/someString 0,1,2,3,4,5,6,7,8,9/5 * * * *'.split(' ');
+        expect(() => conversion(expressions)).to.throw('someString is not a valid step value');
+    });
+
 });