diff --git a/lib/initials/svg.rb b/lib/initials/svg.rb
index b3a340d..3707e26 100644
--- a/lib/initials/svg.rb
+++ b/lib/initials/svg.rb
@@ -21,7 +21,7 @@ def name
 
     def to_s
       svg = [
-        "<svg width='#{size}' height='#{size}'>",
+        "<svg xmlns='http://www.w3.org/2000/svg' width='#{size}' height='#{size}'>",
           shape == :rect ?
             "<rect width='#{size}' height='#{size}' rx='#{size / 32}' ry='#{size / 32}' fill='#{fill}' />"
           :
diff --git a/spec/svg_spec.rb b/spec/svg_spec.rb
index 8a6172c..d2421b7 100644
--- a/spec/svg_spec.rb
+++ b/spec/svg_spec.rb
@@ -83,6 +83,12 @@
       end
     end
 
+    describe "svg tag" do
+      it "has valid xmlns attribute" do
+        expect(subject.to_s).to match(/^<svg xmlns='http:\/\/www.w3.org\/2000\/svg'.+<\/svg>/)
+      end
+    end
+
     describe "size" do
       let(:options) { {size: 512} }