diff --git a/coreneuron/apps/main1.cpp b/coreneuron/apps/main1.cpp
index 2f6530311..fb74df7d0 100644
--- a/coreneuron/apps/main1.cpp
+++ b/coreneuron/apps/main1.cpp
@@ -516,7 +516,6 @@ extern "C" void mk_mech_init(int argc, char** argv) {
     }
 
     // reads mechanism information from bbcore_mech.dat
-
     mk_mech((corenrn_param.datpath).c_str());
 }
 
diff --git a/coreneuron/io/nrn_filehandler.cpp b/coreneuron/io/nrn_filehandler.cpp
index 4318d9bee..40171aec7 100644
--- a/coreneuron/io/nrn_filehandler.cpp
+++ b/coreneuron/io/nrn_filehandler.cpp
@@ -17,7 +17,7 @@ FileHandler::FileHandler(const std::string& filename)
     this->open(filename);
 }
 
-bool FileHandler::file_exist(const std::string& filename) const {
+bool FileHandler::file_exist(const std::string& filename) {
     struct stat buffer;
     return (stat(filename.c_str(), &buffer) == 0);
 }
diff --git a/coreneuron/io/nrn_filehandler.hpp b/coreneuron/io/nrn_filehandler.hpp
index e7d280023..65cab4b97 100644
--- a/coreneuron/io/nrn_filehandler.hpp
+++ b/coreneuron/io/nrn_filehandler.hpp
@@ -61,7 +61,7 @@ class FileHandler {
         return F.fail();
     }
 
-    bool file_exist(const std::string& filename) const;
+    static bool file_exist(const std::string& filename);
 
     /** nothing more to read */
     bool eof();
diff --git a/coreneuron/io/nrn_setup.hpp b/coreneuron/io/nrn_setup.hpp
index 3f5d90a8b..8d1f757b5 100644
--- a/coreneuron/io/nrn_setup.hpp
+++ b/coreneuron/io/nrn_setup.hpp
@@ -117,8 +117,12 @@ inline void* phase_wrapper_w(NrnThread* nt, UserParams& userParams, bool in_memo
                                 ".dat";
 
             // Avoid trying to open the gid_gap.dat file if it doesn't exist when there are no
-            // gap junctions in this gid
-            if (P == gap && !userParams.file_reader[i].file_exist(fname)) {
+            // gap junctions in this gid.
+            // Note that we still need to close `userParams.file_reader[i]`
+            // because files are opened in the order of `gid_1.dat`, `gid_2.dat` and `gid_gap.dat`.
+            // When we open next file, `gid_gap.dat` in this case, we are supposed to close the
+            // handle for `gid_2.dat` even though file doesn't exist.
+            if (P == gap && !FileHandler::file_exist(fname)) {
                 userParams.file_reader[i].close();
             } else {
                 // if no file failed to open or not opened at all
diff --git a/coreneuron/mechanism/mech/cfile/cabvars.h b/coreneuron/mechanism/mech/cfile/cabvars.h
index 846892263..96b90e1b3 100644
--- a/coreneuron/mechanism/mech/cfile/cabvars.h
+++ b/coreneuron/mechanism/mech/cfile/cabvars.h
@@ -22,6 +22,6 @@ static void (*mechanism[])(void) = {/* type will start at 3 */
                                     /* extracellular requires special handling and must be type 5 */
                                     extracell_reg_,
 #endif
-                                    0};
+                                    nullptr};
 
 }  // namespace coreneuron
diff --git a/tests/integration/ring_gap/mod files/halfgap.mod b/tests/integration/ring_gap/mod files/halfgap.mod
old mode 100755
new mode 100644