diff --git a/include/pocketpy/str.h b/include/pocketpy/str.h
index 612b720b..4a4b4ded 100644
--- a/include/pocketpy/str.h
+++ b/include/pocketpy/str.h
@@ -23,7 +23,7 @@ struct Str{
     char* data;
     char _inlined[16];
 
-    const char* _cached_c_str = nullptr;
+    mutable const char* _cached_c_str = nullptr;
 
     bool is_inlined() const { return data == _inlined; }
 
@@ -124,7 +124,7 @@ struct Str{
     Str substr(int start, int len) const;
     Str substr(int start) const;
     char* c_str_dup() const;
-    const char* c_str();
+    const char* c_str() const;
     std::string_view sv() const;
     std::string str() const;
     Str lstrip() const;
diff --git a/src/str.cpp b/src/str.cpp
index d98bb24d..35cb1f2b 100644
--- a/src/str.cpp
+++ b/src/str.cpp
@@ -103,7 +103,7 @@ namespace pkpy {
         return p;
     }
 
-    const char* Str::c_str(){
+    const char* Str::c_str() const{
         if(_cached_c_str == nullptr){
             _cached_c_str = c_str_dup();
         }