diff --git a/include/CoolPropTools.h b/include/CoolPropTools.h
index 38b8ed0f..827b21c8 100644
--- a/include/CoolPropTools.h
+++ b/include/CoolPropTools.h
@@ -117,11 +117,12 @@
     {
         return (path == L"." || path == L"..");
     }
-    inline unsigned long long CalculateDirSize(const std::wstring &path, std::vector<std::wstring> *errVect = NULL, unsigned long long size = 0)
+    inline unsigned long long CalculateDirSize(const std::wstring &path, std::vector<std::wstring> *errVect = NULL)
     {
-        WIN32_FIND_DATA data;
+        unsigned long long size = 0;
+        WIN32_FIND_DATAW data;
         HANDLE sh = NULL;
-        sh = FindFirstFile((path + L"\\*").c_str(), &data);
+        sh = FindFirstFileW((path + L"\\*").c_str(), &data);
 
         if (sh == INVALID_HANDLE_VALUE )
         {
@@ -137,15 +138,15 @@
             if (!IsBrowsePath(data.cFileName))
             {
                 // if found object is ...
-                if ((data.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY) == FILE_ATTRIBUTE_DIRECTORY)
+                if (data.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY)
                     // directory, then search it recursievly
-                    size = CalculateDirSize(path + L"\\" + data.cFileName, NULL, size);
+                    size += CalculateDirSize(path + L"\\" + data.cFileName, NULL);
                 else
                     // otherwise get object size and add it to directory size
-                    size += (unsigned long long) (data.nFileSizeHigh * (MAXDWORD ) + data.nFileSizeLow);
+                    size += data.nFileSizeHigh * (unsigned long long)(MAXDWORD) + data.nFileSizeLow;
             }
 
-        } while (FindNextFile(sh, &data)); // do
+        } while (FindNextFileW(sh, &data)); // do
 
         FindClose(sh);
 
diff --git a/src/CoolPropTools.cpp b/src/CoolPropTools.cpp
index 559cb560..18c83b6a 100644
--- a/src/CoolPropTools.cpp
+++ b/src/CoolPropTools.cpp
@@ -17,7 +17,7 @@
 #include <stdint.h>
 #include <iostream>
 
-static double ftw_summer; // An evil global variable for the ftw function
+static thread_local unsigned long long ftw_summer; // An evil global variable for the ftw function
 int ftw_function(const char *fpath, const struct stat *sb, int tflag, struct FTW *ftwbuf){
    ftw_summer += sb->st_size;
    return 0;           /* To tell nftw() to continue */