diff --git a/lib/git_stats/core_extensions/hash.rb b/lib/git_stats/core_extensions/hash.rb
index 995fb370a..4fd5a947a 100644
--- a/lib/git_stats/core_extensions/hash.rb
+++ b/lib/git_stats/core_extensions/hash.rb
@@ -1,5 +1,6 @@
 class Hash
   def to_key_indexed_array
-    self.inject([]) { |acc, (k, v)| acc[k] = v; acc }
+    raise ArgumentError.new('all the keys must be numbers to convert to key indexed array') unless all? { |k, v| k.is_a? Numeric }
+    inject([]) { |acc, (k, v)| acc[k] = v; acc }
   end
 end
\ No newline at end of file
diff --git a/spec/hash_extension_spec.rb b/spec/hash_extension_spec.rb
new file mode 100644
index 000000000..4afd44e17
--- /dev/null
+++ b/spec/hash_extension_spec.rb
@@ -0,0 +1,15 @@
+require 'spec_helper'
+
+describe Hash do
+  context 'to_key_indexed_array' do
+    it 'should convert hash to array using keys as indexes' do
+      hash = {1 => 'x', 2 => 1, 5 => 'a'}
+      hash.to_key_indexed_array.should == [nil, 'x', 1, nil, nil, 'a']
+    end
+
+    it 'should throw exception if not all of the keys are numbers' do
+      hash = {1 => 'x', 'b' => 2}
+      expect { hash.to_key_indexed_array }.to raise_error(ArgumentError)
+    end
+  end
+end
\ No newline at end of file