diff --git a/doc/src/Fortran.rst b/doc/src/Fortran.rst
index eaef02aa71..10947bfef9 100644
--- a/doc/src/Fortran.rst
+++ b/doc/src/Fortran.rst
@@ -47,6 +47,14 @@ Fortran code in order to uses the Fortran interface.
 A working example can be found together with equivalent examples in C and
 C++ in the ``examples/COUPLE/simple`` folder of the LAMMPS distribution.
 
+.. admonitions:: Fortran compiler compatibility
+
+   A fully Fortran 2003 compatible Fortran compiler is required.
+   This means that currently only GNU Fortran 9 and later are
+   compatible and thus the default compilers of Red Hat or CentOS 7
+   and Ubuntu 18.04 LTS and not compatible.  Either newer compilers
+   need to be installed or the Linux updated.
+
 .. versionchanged:: TBD
 
 .. note::
diff --git a/examples/COUPLE/simple/CMakeLists.txt b/examples/COUPLE/simple/CMakeLists.txt
index 46eedcf2d4..14c253e5a1 100644
--- a/examples/COUPLE/simple/CMakeLists.txt
+++ b/examples/COUPLE/simple/CMakeLists.txt
@@ -52,32 +52,37 @@ if(LAMMPS_SOURCE_DIR)
 
   include(CheckLanguage)
   check_language(Fortran)
-
-  if(CMAKE_Fortran_COMPILER AND MPI_Fortran_FOUND AND MPI_Fortran_HAVE_F90_MODULE)
+  if(CMAKE_Fortran_COMPILER)
     enable_language(Fortran)
+    # need to check for MPI again to include Fortran components, since Fortran wasn't enabled before
+    find_package(MPI QUIET COMPONENTS Fortran)
 
-    if(NOT CMAKE_Fortran_COMPILER_ID)
-      message(STATUS "Skipping Tests for the LAMMPS Fortran Module: cannot identify Fortran compiler")
+    if(NOT (MPI_Fortran_FOUND AND MPI_Fortran_HAVE_F90_MODULE))
+      message(STATUS "Insufficient MPI Fortran support. Skipping building simpleF90")
       return()
     endif()
 
     # GNU Fortran 4.8.x on RHEL/CentOS 7.x is not sufficient to compile the Fortran module.
+    # GNU Fortran 7.x on Ubuntu 18.04LTS fails as well.
     # Work around flang being detected as GNU
     get_filename_component(_tmp_fc ${CMAKE_Fortran_COMPILER} NAME)
-    if((CMAKE_Fortran_COMPILER_ID STREQUAL "GNU") AND (CMAKE_Fortran_COMPILER_VERSION VERSION_LESS 6.0) AND NOT (_tmp_fc STREQUAL "flang"))
-      message(FATAL_ERROR "Need GNU Fortran compiler version 6.x or later for unit testing")
+    if((CMAKE_Fortran_COMPILER_ID STREQUAL "GNU") AND (CMAKE_Fortran_COMPILER_VERSION VERSION_LESS 9.0) AND NOT (_tmp_fc STREQUAL "flang"))
+      message(STATUS "Need GNU Fortran compiler version 9.x or later for LAMMPS Fortran module")
+      return()
     endif()
 
     get_filename_component(LAMMPS_FORTRAN_MODULE ${LAMMPS_SOURCE_DIR}/../fortran/lammps.f90 ABSOLUTE)
     add_executable(simpleF90 ${LAMMPS_FORTRAN_MODULE} simple.f90)
     target_link_libraries(simpleF90 PRIVATE MPI::MPI_Fortran LAMMPS::lammps)
-    if(CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
-      target_compile_options(simpleF90 PRIVATE -fallow-argument-mismatch)
-    endif()
 
     get_filename_component(_tmp_fc ${CMAKE_Fortran_COMPILER} NAME)
     if (_tmp_fc STREQUAL "flang")
       target_link_libraries(simpleF90 PRIVATE gfortran)
     endif()
+    include(CheckFortranCompilerFlag)
+    check_fortran_compiler_flag(-fallow-argument-mismatch HAS_MISMATCH)
+    if(HAS_MISMATCH)
+      target_compile_options(simpleF90 PRIVATE -fallow-argument-mismatch)
+    endif()
   endif()
 endif()
diff --git a/fortran/README b/fortran/README
index 6a19cd7dc2..2da71b2dc2 100644
--- a/fortran/README
+++ b/fortran/README
@@ -1,6 +1,7 @@
 This directory contains Fortran code that acts as an interface to LAMMPS as a
 library and allows the LAMMPS library interface to be invoked from Fortran
-code.  It requires a Fortran compiler that supports the Fortran 2003 standard.
+code.  It requires a Fortran compiler that supports the Fortran 2003 standard
+sufficiently well. For example GNU Fortran 9.x or later is currently required.
 
 This interface is based on and supersedes the previous Fortran interfaces
 in the examples/COUPLE/fortran* folders, but it is fully supported by the
diff --git a/unittest/c-library/CMakeLists.txt b/unittest/c-library/CMakeLists.txt
index 9ca83d1741..7543e21948 100644
--- a/unittest/c-library/CMakeLists.txt
+++ b/unittest/c-library/CMakeLists.txt
@@ -88,8 +88,7 @@ if(BUILD_MPI)
   add_test(NAME RunCoupleSimpleCC
     COMMAND $<TARGET_FILE:simpleCC> 1 ${LAMMPS_DIR}/examples/COUPLE/simple/in.lj)
 
-  find_package(MPI QUIET)
-  if(CMAKE_Fortran_COMPILER AND MPI_Fortran_FOUND AND MPI_Fortran_HAVE_F90_MODULE)
+  if($<TARGET_EXISTS:simpleF90>)
     add_test(NAME RunCoupleSimpleF90
       COMMAND $<TARGET_FILE:simpleF90> 1 ${LAMMPS_DIR}/examples/COUPLE/simple/in.lj)
     set_tests_properties(RunCoupleSimpleF90 PROPERTIES
diff --git a/unittest/fortran/CMakeLists.txt b/unittest/fortran/CMakeLists.txt
index cec38ae55c..138af65f18 100644
--- a/unittest/fortran/CMakeLists.txt
+++ b/unittest/fortran/CMakeLists.txt
@@ -14,10 +14,12 @@ if(CMAKE_Fortran_COMPILER)
     message(STATUS "Skipping Tests for the LAMMPS Fortran Module: cannot identify Fortran compiler")
     return()
   endif()
-  # GNU Fortran 4.8.x on RHEL/CentOS 7.x is not sufficient to compile the Fortran module. Work around flang being detected as GNU
+  # GNU Fortran 4.8.x on RHEL/CentOS 7.x is not sufficient to compile the Fortran module.
+  # Also GNU Fortran 7.x on Ubuntu18.04LTS is not sufficient. Work around flang being detected as GNU
   get_filename_component(_tmp_fc ${CMAKE_Fortran_COMPILER} NAME)
-  if((CMAKE_Fortran_COMPILER_ID STREQUAL "GNU") AND (CMAKE_Fortran_COMPILER_VERSION VERSION_LESS 6.0) AND NOT (_tmp_fc STREQUAL "flang"))
-    message(FATAL_ERROR "Need GNU Fortran compiler version 6.x or later for unit testing")
+  if((CMAKE_Fortran_COMPILER_ID STREQUAL "GNU") AND (CMAKE_Fortran_COMPILER_VERSION VERSION_LESS 9.0) AND NOT (_tmp_fc STREQUAL "flang"))
+    message(STATUS "Need GNU Fortran compiler version 9.x or later for Fortran unit testing. Skipping...")
+    return()
   endif()
 
   get_filename_component(LAMMPS_FORTRAN_MODULE ${LAMMPS_SOURCE_DIR}/../fortran/lammps.f90 ABSOLUTE)