diff --git a/.travis.yml b/.travis.yml
index bf4ce2c1..a18efefc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,6 +6,9 @@ matrix:
     - os: osx
       osx_image: xcode9
       env: SWIFT_VERSION=4.0
+    - os: osx
+      osx_image: xcode9.1
+      env: SWIFT_VERSION=4.0
     - os: linux
       env: SWIFT_VERSION=3.1.1
     - os: linux
diff --git a/Sources/Lexer.swift b/Sources/Lexer.swift
index b0ad8f98..5bd590da 100644
--- a/Sources/Lexer.swift
+++ b/Sources/Lexer.swift
@@ -10,7 +10,7 @@ struct Lexer {
       guard string.characters.count > 4 else { return "" }
       let start = string.index(string.startIndex, offsetBy: 2)
       let end = string.index(string.endIndex, offsetBy: -2)
-      return string[start..<end].trim(character: " ")
+      return String(string[start..<end]).trim(character: " ")
     }
 
     if string.hasPrefix("{{") {
@@ -149,6 +149,6 @@ extension String {
   func trim(character: Character) -> String {
     let first = findFirstNot(character: character) ?? startIndex
     let last = findLastNot(character: character) ?? endIndex
-    return self[first..<last]
+    return String(self[first..<last])
   }
 }