diff --git a/flang/runtime/external-unit.cpp b/flang/runtime/external-unit.cpp
index 328c994a180b1..8009151a8a370 100644
--- a/flang/runtime/external-unit.cpp
+++ b/flang/runtime/external-unit.cpp
@@ -65,10 +65,17 @@ ExternalFileUnit *ExternalFileUnit::LookUpOrCreateAnonymous(int unit,
   bool exists{false};
   ExternalFileUnit *result{GetUnitMap().LookUpOrCreate(unit, handler, exists)};
   if (result && !exists) {
-    result->OpenAnonymousUnit(
-        dir == Direction::Input ? OpenStatus::Unknown : OpenStatus::Replace,
-        Action::ReadWrite, Position::Rewind, Convert::Unknown, handler);
-    result->isUnformatted = isUnformatted;
+    if (!result->OpenAnonymousUnit(
+            dir == Direction::Input ? OpenStatus::Unknown : OpenStatus::Replace,
+            Action::ReadWrite, Position::Rewind, Convert::Unknown, handler)) {
+      // fort.N isn't a writable file
+      if (ExternalFileUnit * closed{LookUpForClose(result->unitNumber())}) {
+        closed->DestroyClosed();
+      }
+      result = nullptr;
+    } else {
+      result->isUnformatted = isUnformatted;
+    }
   }
   return result;
 }
@@ -183,7 +190,7 @@ bool ExternalFileUnit::OpenUnit(Fortran::common::optional<OpenStatus> status,
   return impliedClose;
 }
 
-void ExternalFileUnit::OpenAnonymousUnit(
+bool ExternalFileUnit::OpenAnonymousUnit(
     Fortran::common::optional<OpenStatus> status,
     Fortran::common::optional<Action> action, Position position,
     Convert convert, IoErrorHandler &handler) {
@@ -193,6 +200,7 @@ void ExternalFileUnit::OpenAnonymousUnit(
   std::snprintf(path.get(), pathMaxLen, "fort.%d", unitNumber_);
   OpenUnit(status, action, position, std::move(path), std::strlen(path.get()),
       convert, handler);
+  return IsConnected();
 }
 
 void ExternalFileUnit::CloseUnit(CloseStatus status, IoErrorHandler &handler) {
diff --git a/flang/runtime/pseudo-unit.cpp b/flang/runtime/pseudo-unit.cpp
index 70e70eec6a77e..526afd11d916e 100644
--- a/flang/runtime/pseudo-unit.cpp
+++ b/flang/runtime/pseudo-unit.cpp
@@ -65,7 +65,7 @@ bool ExternalFileUnit::OpenUnit(Fortran::common::optional<OpenStatus> status,
   handler.Crash("%s: unsupported", RT_PRETTY_FUNCTION);
 }
 
-void ExternalFileUnit::OpenAnonymousUnit(Fortran::common::optional<OpenStatus>,
+bool ExternalFileUnit::OpenAnonymousUnit(Fortran::common::optional<OpenStatus>,
     Fortran::common::optional<Action>, Position, Convert convert,
     IoErrorHandler &handler) {
   handler.Crash("%s: unsupported", RT_PRETTY_FUNCTION);
diff --git a/flang/runtime/unit.h b/flang/runtime/unit.h
index abd535fd0381d..83f839e205a48 100644
--- a/flang/runtime/unit.h
+++ b/flang/runtime/unit.h
@@ -134,7 +134,7 @@ class ExternalFileUnit : public ConnectionState,
   RT_API_ATTRS bool OpenUnit(Fortran::common::optional<OpenStatus>,
       Fortran::common::optional<Action>, Position, OwningPtr<char> &&path,
       std::size_t pathLength, Convert, IoErrorHandler &);
-  RT_API_ATTRS void OpenAnonymousUnit(Fortran::common::optional<OpenStatus>,
+  RT_API_ATTRS bool OpenAnonymousUnit(Fortran::common::optional<OpenStatus>,
       Fortran::common::optional<Action>, Position, Convert, IoErrorHandler &);
   RT_API_ATTRS void CloseUnit(CloseStatus, IoErrorHandler &);
   RT_API_ATTRS void DestroyClosed();