diff --git a/Example/Package.resolved b/Example/Package.resolved
index e7b9ecc..8bd756a 100644
--- a/Example/Package.resolved
+++ b/Example/Package.resolved
@@ -2,7 +2,7 @@
   "object": {
     "pins": [
       {
-        "package": "cmark-gfm",
+        "package": "cmark",
         "repositoryURL": "https://github.com/brokenhandsio/cmark-gfm.git",
         "state": {
           "branch": null,
@@ -65,7 +65,7 @@
         }
       },
       {
-        "package": "Swim",
+        "package": "HTML",
         "repositoryURL": "https://github.com/robb/Swim",
         "state": {
           "branch": null,
diff --git a/Sources/Saga/String+Slugify.swift b/Sources/Saga/String+Slugify.swift
index 9964e30..0c20bb1 100644
--- a/Sources/Saga/String+Slugify.swift
+++ b/Sources/Saga/String+Slugify.swift
@@ -1,11 +1,13 @@
 import Foundation
 
+private let allowedCharacters = CharacterSet(charactersIn: "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-_")
+
 public extension CustomStringConvertible {
   var slugified: String {
     return self.description
-      .components(separatedBy: CharacterSet.alphanumerics.union(.whitespaces).inverted)
-      .joined(separator: "")
-      .replacingOccurrences(of: " ", with: "-")
+      .components(separatedBy: allowedCharacters.inverted)
+      .filter { $0 != "" }
+      .joined(separator: "-")
       .lowercased()
   }
 }
diff --git a/Tests/SagaTests/SagaTests.swift b/Tests/SagaTests/SagaTests.swift
index 52d1447..508a3f1 100644
--- a/Tests/SagaTests/SagaTests.swift
+++ b/Tests/SagaTests/SagaTests.swift
@@ -263,6 +263,9 @@ final class SagaTests: XCTestCase {
     XCTAssertEqual("one two".slugified, "one-two")
     XCTAssertEqual("One Two".slugified, "one-two")
     XCTAssertEqual("One! .Two@".slugified, "one-two")
+    XCTAssertEqual("one-two".slugified, "one-two")
+    XCTAssertEqual("one_two".slugified, "one_two")
+    XCTAssertEqual("ONE-TWO".slugified, "one-two")
   }
 
   static var allTests = [