diff --git a/Sources/Cheetah/InternalHelpers.swift b/Sources/Cheetah/InternalHelpers.swift
index 468cd19..98c145c 100644
--- a/Sources/Cheetah/InternalHelpers.swift
+++ b/Sources/Cheetah/InternalHelpers.swift
@@ -31,3 +31,15 @@ extension UInt16 {
         ]
     }
 }
+
+extension String {
+    #if swift(>=4.0)
+    internal func toCharacterSequence() -> String {
+        return self
+    }
+    #else
+    internal func toCharacterSequence() -> CharacterView {
+        return self.characters
+    }
+    #endif    
+}
diff --git a/Sources/Cheetah/Parser.swift b/Sources/Cheetah/Parser.swift
index 2ae4de4..5daefdb 100644
--- a/Sources/Cheetah/Parser.swift
+++ b/Sources/Cheetah/Parser.swift
@@ -216,7 +216,7 @@ public struct JSON {
         func parseFracture(prefix: String, autoNegate: Bool = false) throws -> Double? {
             var prefix = prefix
             
-            if negate && autoNegate && prefix.characters.first != "-" {
+            if negate && autoNegate && prefix.toCharacterSequence().first != "-" {
                 prefix = "-" + prefix
             }
             
@@ -228,7 +228,7 @@ public struct JSON {
                 position += 1
             }
             
-            guard number.characters.count > 0 else {
+            guard number.toCharacterSequence().count > 0 else {
                 return nil
             }
             
diff --git a/Sources/Cheetah/Value.swift b/Sources/Cheetah/Value.swift
index 969f9ec..b4481ce 100644
--- a/Sources/Cheetah/Value.swift
+++ b/Sources/Cheetah/Value.swift
@@ -100,7 +100,7 @@ extension String: Value {
             case 0x00...0x1F:
                 buffer.append(contentsOf: "\\u".utf8)
                 let str = String(char, radix: 16, uppercase: true)
-                if str.characters.count == 1 {
+                if str.toCharacterSequence().count == 1 {
                     buffer.append(contentsOf: "000\(str)".utf8)
                 } else {
                     buffer.append(contentsOf: "00\(str)".utf8)