diff --git a/echion/strings.h b/echion/strings.h
index 64005206..44231151 100644
--- a/echion/strings.h
+++ b/echion/strings.h
@@ -211,12 +211,12 @@ class StringTable : public std::unordered_map<uintptr_t, std::string>
     }
 #endif  // UNWIND_NATIVE_DISABLE
 
-    [[nodiscard]] inline Result<std::reference_wrapper<std::string>> lookup(Key key)
+    [[nodiscard]] inline Result<std::reference_wrapper<const std::string>> lookup(Key key) const
     {
         const std::lock_guard<std::mutex> lock(table_lock);
 
-        auto it = this->find(key);
-        if (it == this->end())
+        const auto it = this->find(key);
+        if (it == this->cend())
             return ErrorKind::LookupError;
 
         return std::ref(it->second);
@@ -230,7 +230,7 @@ class StringTable : public std::unordered_map<uintptr_t, std::string>
     };
 
 private:
-    std::mutex table_lock;
+    mutable std::mutex table_lock;
 };
 
 // We make this a reference to a heap-allocated object so that we can avoid