diff --git a/src/libtorchaudio/sox/utils.cpp b/src/libtorchaudio/sox/utils.cpp
index 80747816c7..94748c5209 100644
--- a/src/libtorchaudio/sox/utils.cpp
+++ b/src/libtorchaudio/sox/utils.cpp
@@ -195,7 +195,7 @@ torch::Tensor convert_to_tensor(
 }
 
 const std::string get_filetype(const std::string& path) {
-  std::string ext = path.substr(path.find_last_of(".") + 1);
+  std::string ext = path.substr(path.find_last_of('.') + 1);
   std::transform(ext.begin(), ext.end(), ext.begin(), ::tolower);
   return ext;
 }