diff --git a/lib/twine/runner.rb b/lib/twine/runner.rb
index 3b91d19..14ef211 100644
--- a/lib/twine/runner.rb
+++ b/lib/twine/runner.rb
@@ -211,7 +211,7 @@ module Twine
         errors << "None of your strings have tags."
       elsif keys_without_tags.length > 0
         error_body = keys_without_tags.to_a.join("\n  ")
-        errors << "Found strings(s) without tags:\n  #{error_body}"
+        errors << "Found strings without tags:\n  #{error_body}"
       end
 
       if errors.length > 0
diff --git a/test/test_validate_strings_file.rb b/test/test_validate_strings_file.rb
new file mode 100644
index 0000000..f6274c0
--- /dev/null
+++ b/test/test_validate_strings_file.rb
@@ -0,0 +1,45 @@
+require 'command_test_case'
+
+class TestValidateStringsFile < CommandTestCase
+  def setup
+    super
+    @options = { strings_file: 'input.txt' }
+
+    @twine_file = build_twine_file 'en' do
+      add_section 'Section 1' do
+        add_row key1: 'value1', tags: ['tag1']
+        add_row key2: 'value2', tags: ['tag1']
+      end
+
+      add_section 'Section 2' do
+        add_row key3: 'value3', tags: ['tag1', 'tag2']
+        add_row key4: 'value4', tags: ['tag2']
+      end
+    end
+  end
+
+  def random_row
+    @twine_file.strings_map[@twine_file.strings_map.keys.sample]
+  end
+
+  def test_recognizes_valid_file
+    Twine::Runner.new(@options, @twine_file).validate_strings_file
+    assert_equal "input.txt is valid.\n", Twine::stdout.string
+  end
+
+  def test_reports_duplicate_keys
+    @twine_file.sections[0].rows << random_row
+
+    assert_raises Twine::Error do
+      Twine::Runner.new(@options, @twine_file).validate_strings_file
+    end
+  end
+
+  def test_reports_missing_tags
+    random_row.tags.clear
+
+    assert_raises Twine::Error do
+      Twine::Runner.new(@options, @twine_file).validate_strings_file
+    end
+  end
+end