diff --git a/Sources/Geohash/Geohash.swift b/Sources/Geohash/Geohash.swift
index b8505dd..adc4b9f 100644
--- a/Sources/Geohash/Geohash.swift
+++ b/Sources/Geohash/Geohash.swift
@@ -144,7 +144,7 @@ public extension CLLocationCoordinate2D {
 #endif
 
 public extension Geohash {
-    private static var base32 = "0123456789bcdefghjkmnpqrstuvwxyz"
+    private static let base32 = "0123456789bcdefghjkmnpqrstuvwxyz"
     enum Direction: String {
         case n, e, s, w