diff --git a/lib/twine/runner.rb b/lib/twine/runner.rb
index 3b91d19..3b9ec49 100644
--- a/lib/twine/runner.rb
+++ b/lib/twine/runner.rb
@@ -184,40 +184,41 @@ module Twine
       all_keys = Set.new
       duplicate_keys = Set.new
       keys_without_tags = Set.new
-      errors = []
+      invalid_keys = Set.new
+      valid_key_regex = /^[A-Za-z0-9_.]+$/
 
       @strings.sections.each do |section|
         section.rows.each do |row|
           total_strings += 1
 
-          if all_keys.include? row.key
-            duplicate_keys.add(row.key)
-          else
-            all_keys.add(row.key)
-          end
+          duplicate_keys.add(row.key) if all_keys.include? row.key
+          all_keys.add(row.key)
 
-          if row.tags == nil || row.tags.length == 0
-            keys_without_tags.add(row.key)
-          end
+          keys_without_tags.add(row.key) if row.tags == nil or row.tags.length == 0
+
+          invalid_keys << row.key unless row.key =~ valid_key_regex
         end
       end
 
-      if duplicate_keys.length > 0
-        error_body = duplicate_keys.to_a.join("\n  ")
-        errors << "Found duplicate string key(s):\n  #{error_body}"
+      errors = []
+      join_keys = lambda { |set| set.map { |k| "  " + k }.join("\n") }
+
+      unless duplicate_keys.empty?
+        errors << "Found duplicate string key(s):\n#{join_keys.call(duplicate_keys)}"
       end
 
       if keys_without_tags.length == total_strings
         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#{join_keys.call(keys_without_tags)}"
       end
 
-      if errors.length > 0
-        raise Twine::Error.new errors.join("\n\n")
+      unless invalid_keys.empty?
+        errors << "Found key(s) with invalid characters:\n#{join_keys.call(invalid_keys)}"
       end
 
+      raise Twine::Error.new errors.join("\n\n") unless errors.empty?
+
       Twine::stdout.puts "#{@options[:strings_file]} is valid."
     end
 
diff --git a/test/test_validate_strings_file.rb b/test/test_validate_strings_file.rb
new file mode 100644
index 0000000..fcb6a27
--- /dev/null
+++ b/test/test_validate_strings_file.rb
@@ -0,0 +1,55 @@
+# encoding: utf-8
+
+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
+
+  def test_reports_invalid_characters_in_keys
+    random_row.key[0] = "!?;:,^`´'\"\\|/(){}[]~-+*=#$%".chars.to_a.sample
+
+    assert_raises Twine::Error do
+      Twine::Runner.new(@options, @twine_file).validate_strings_file
+    end
+  end
+end