diff --git a/csv.h b/csv.h
index 0adf9c0..9a1919a 100644
--- a/csv.h
+++ b/csv.h
@@ -407,7 +407,7 @@ class LineReader {
 
   void set_file_name(const char *file_name) {
     if (file_name != nullptr) {
-      strncpy(this->file_name, file_name, sizeof(this->file_name));
+      strncpy(this->file_name, file_name, sizeof(this->file_name) - 1);
       this->file_name[sizeof(this->file_name) - 1] = '\0';
     } else {
       this->file_name[0] = '\0';