diff --git a/src/util/file_util.cpp b/src/util/file_util.cpp
index 0e8cfc80ddf..e4a96bcf0ca 100644
--- a/src/util/file_util.cpp
+++ b/src/util/file_util.cpp
@@ -136,10 +136,8 @@ std::string concat_dir_file(
   const std::string &file_name)
 {
   #ifdef _WIN32
-  return  (file_name.size()>1 &&
-           file_name[0]!='/' &&
-           file_name[1]!=':') ?
-           file_name : directory+"\\"+file_name;
+  return (file_name.size() > 1 && file_name[0] != '/' && file_name[1] == ':') ?
+          file_name : directory + "\\" + file_name;
   #else
   return (!file_name.empty() && file_name[0]=='/') ?
           file_name : directory+"/"+file_name;
diff --git a/unit/util/file_util.cpp b/unit/util/file_util.cpp
index 8cf4c5bd6f1..2b4bc2055c4 100644
--- a/unit/util/file_util.cpp
+++ b/unit/util/file_util.cpp
@@ -14,6 +14,27 @@ Author: Daniel Kroening
 
 #include <fstream>
 
+TEST_CASE("concat_dir_file functionality", "[core][util][file_util]")
+{
+  temp_dirt temp_dir("testXXXXXX");
+  const std::string path = concat_dir_file(temp_dir.path, "bla.txt");
+
+  REQUIRE(path.size() > temp_dir.path.size() + std::string("bla.txt").size());
+  #ifdef _WIN32
+  REQUIRE(path.find('\\') != std::string::npos);
+  #else
+  REQUIRE(path.find('/') != std::string::npos);
+  #endif
+
+  #ifdef _WIN32
+  const std::string qualified_path = "z:\some\path\foo.txt";
+  #else
+  const std::string qualified_path = "/some/path/foo.txt";
+  #endif
+  const std::string path2 = concat_dir_file(temp_dir.path, qualified_path);
+  REQUIRE(path2 == qualified_path);
+}
+
 TEST_CASE("is_directory functionality", "[core][util][file_util]")
 {
   temp_dirt temp_dir("testXXXXXX");