diff --git a/Sources/Common/DecodingHuffmanTree.swift b/Sources/Common/DecodingHuffmanTree.swift
index 2a9ff316..c5052f2d 100644
--- a/Sources/Common/DecodingHuffmanTree.swift
+++ b/Sources/Common/DecodingHuffmanTree.swift
@@ -8,9 +8,9 @@ import BitByteData
 
 class DecodingHuffmanTree {
 
-    private var bitReader: BitReader
+    private let bitReader: BitReader
 
-    private var tree: [Int]
+    private let tree: [Int]
     private let leafCount: Int
 
     /// `lengths` don't have to be properly sorted, but there must not be any 0 code lengths.
@@ -22,7 +22,7 @@ class DecodingHuffmanTree {
 
         // Calculate maximum amount of leaves possible in a tree.
         self.leafCount = 1 << (sortedLengths.last!.codeLength + 1)
-        self.tree = Array(repeating: -1, count: leafCount)
+        var tree = Array(repeating: -1, count: leafCount)
 
         // Calculates symbols for each length in 'sortedLengths' array and put them in the tree.
         var loopBits = -1
@@ -46,8 +46,9 @@ class DecodingHuffmanTree {
                 index = bit == 0 ? 2 * index + 1 : 2 * index + 2
                 treeCode >>= 1
             }
-            self.tree[index] = length.symbol
+            tree[index] = length.symbol
         }
+        self.tree = tree
     }
 
     func findNextSymbol() -> Int {
diff --git a/Sources/Common/EncodingHuffmanTree.swift b/Sources/Common/EncodingHuffmanTree.swift
index 3ad5f1ca..f7bd5627 100644
--- a/Sources/Common/EncodingHuffmanTree.swift
+++ b/Sources/Common/EncodingHuffmanTree.swift
@@ -8,9 +8,9 @@ import BitByteData
 
 class EncodingHuffmanTree {
 
-    private var bitWriter: BitWriter
+    private let bitWriter: BitWriter
 
-    private var codingIndices: [[Int]]
+    private let codingIndices: [[Int]]
 
     /// `lengths` don't have to be properly sorted, but there must not be any 0 code lengths.
     /// If `reverseCodes` is true, then bit order of tree codes will be reversed. Necessary for Deflate.
@@ -20,7 +20,7 @@ class EncodingHuffmanTree {
         // Sort `lengths` array to calculate canonical Huffman code.
         let sortedLengths = lengths.sorted()
 
-        self.codingIndices = Array(repeating: [-1, -1], count: sortedLengths.count)
+        var codingIndices = Array(repeating: [-1, -1], count: sortedLengths.count)
 
         // Calculates symbols for each length in 'sortedLengths' array and put them in the tree.
         var loopBits = -1
@@ -36,8 +36,9 @@ class EncodingHuffmanTree {
             }
             // Then we reverse bit order of the symbol, if necessary.
             let treeCode = reverseCodes ? symbol.reversed(bits: loopBits) : symbol
-            self.codingIndices[length.symbol] = [treeCode, bits]
+            codingIndices[length.symbol] = [treeCode, bits]
         }
+        self.codingIndices = codingIndices
     }
 
     func code(symbol: Int) {