diff --git a/src/convert-expression/step-values-conversion.js b/src/convert-expression/step-values-conversion.js
index 1f448f4..b94996a 100644
--- a/src/convert-expression/step-values-conversion.js
+++ b/src/convert-expression/step-values-conversion.js
@@ -9,7 +9,7 @@ module.exports = (() => {
             if(isStepValue){
                 var baseDivider = match[2];
                 if(isNaN(baseDivider)){
-                    throw baseDivider + 'is not a valid step value';
+                    throw baseDivider + ' is not a valid step value';
                 }
                 var values = match[1].split(',');
                 var setpValues = [];
diff --git a/test/convert-expression/step-values-conversion-test.js b/test/convert-expression/step-values-conversion-test.js
index 6016630..88c73d4 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');
 var conversion = require('../../src/convert-expression/step-values-conversion');
 
 describe('step-values-conversion.js', () => {
-    it('shuld convert step values', () => {
+    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');
+    });
+
 });