diff --git a/lib/jekyll/site.rb b/lib/jekyll/site.rb
index 9492b693..66136541 100644
--- a/lib/jekyll/site.rb
+++ b/lib/jekyll/site.rb
@@ -424,7 +424,16 @@ module Jekyll
     private
     def configure_theme
       self.theme = nil
-      self.theme = Jekyll::Theme.new(config["theme"]) if config["theme"]
+      return if config["theme"].nil?
+
+      self.theme =
+        if config["theme"].is_a?(String)
+          Jekyll::Theme.new(config["theme"])
+        else
+          Jekyll.logger.warn "Theme:", "value of 'theme' in config should be " \
+          "String to use gem-based themes, but got #{config["theme"].class}"
+          nil
+        end
     end
 
     private
diff --git a/test/test_site.rb b/test/test_site.rb
index f09d94b9..57b06871 100644
--- a/test/test_site.rb
+++ b/test/test_site.rb
@@ -489,6 +489,34 @@ class TestSite < JekyllUnitTest
       end
     end
 
+    context "when setting theme" do
+      should "set no theme if config is not set" do
+        expect($stderr).not_to receive(:puts)
+        expect($stdout).not_to receive(:puts)
+        site = fixture_site({ "theme" => nil })
+        assert_nil site.theme
+      end
+
+      should "set no theme if config is a hash" do
+        output = capture_output do
+          site = fixture_site({ "theme" => {} })
+          assert_nil site.theme
+        end
+        expected_msg = "Theme: value of 'theme' in config should be String " \
+          "to use gem-based themes, but got Hash\n"
+        assert output.end_with?(expected_msg),
+          "Expected #{output.inspect} to end with #{expected_msg.inspect}"
+      end
+
+      should "set a theme if the config is a string" do
+        expect($stderr).not_to receive(:puts)
+        expect($stdout).not_to receive(:puts)
+        site = fixture_site({ "theme" => "test-theme" })
+        assert_instance_of Jekyll::Theme, site.theme
+        assert_equal "test-theme", site.theme.name
+      end
+    end
+
     context "with liquid profiling" do
       setup do
         @site = Site.new(site_configuration("profile" => true))