diff --git a/index.js b/index.js
index 9e3b3ff..39af778 100644
--- a/index.js
+++ b/index.js
@@ -48,10 +48,6 @@ let mode = numbers => {
 };
 
 let standardDeviation = numbers => {
-    if (!Array.isArray(numbers)) {
-        return null;
-    }
-
     return Math.sqrt(variance(numbers));
 };
 
diff --git a/test/test.js b/test/test.js
index 29bbe1c..8776bb7 100644
--- a/test/test.js
+++ b/test/test.js
@@ -7,13 +7,33 @@ var stats = require('../index');
 
 describe('#stats', function() {
     it('Median', function() {
-        var result = stats.median([1,2,3]);
+        var result = stats.median([1, 2, 3]);
         expect(result).to.equal(2);
     });
-    it('Not an array error', function() {
+    it('Median with even length array', function() {
+        var result = stats.median([1, 2, 3, 4]);
+        expect(result).to.equal(2.5);
+    });
+    it('Not an array error median', function() {
         var result = stats.median(1);
         expect(result).to.equal(null);
     });
+    it('Not an array error mean', function() {
+        var result = stats.mean(1);
+        expect(result).to.equal(null);
+    });
+    it('Not an array error mode', function() {
+        var result = stats.mode(1);
+        expect(result).to.equal(null);
+    });
+    it('Not an array error variance', function() {
+        var result = stats.variance(1);
+        expect(result).to.equal(null);
+    });
+    it('Not an array error harmonic mean', function() {
+        var result = stats.harmonicMean(1);
+        expect(result).to.equal(null);
+    });
     it('Mode', function() {
         var result = stats.mode([1, 2, 1, 3]);
         expect(result[0]).to.equal(1);