diff --git a/Modules/Compiler/GNU-Fortran.cmake b/Modules/Compiler/GNU-Fortran.cmake
index dfd79274d..e9c8a594a 100644
--- a/Modules/Compiler/GNU-Fortran.cmake
+++ b/Modules/Compiler/GNU-Fortran.cmake
@@ -8,5 +8,8 @@ set(CMAKE_Fortran_FORMAT_FREE_FLAG "-ffree-form")
 set(CMAKE_Fortran_FLAGS_MINSIZEREL_INIT "-Os")
 set(CMAKE_Fortran_FLAGS_RELEASE_INIT "-O3")
 
+# No -isystem for Fortran because it will not find .mod files.
+unset(CMAKE_INCLUDE_SYSTEM_FLAG_Fortran)
+
 # Fortran-specific feature flags.
 set(CMAKE_Fortran_MODDIR_FLAG -J)