diff --git a/Modules/FortranCInterface.cmake b/Modules/FortranCInterface.cmake
index 812e3198a..a7d557136 100644
--- a/Modules/FortranCInterface.cmake
+++ b/Modules/FortranCInterface.cmake
@@ -248,9 +248,9 @@ function(FortranCInterface_VERIFY)
       VerifyFortranC
       CMAKE_FLAGS -DVERIFY_CXX=${verify_cxx}
                   -DCMAKE_VERBOSE_MAKEFILE=ON
-                 "-DCMAKE_C_FLAGS=${CMAKE_C_FLAGS}"
-                 "-DCMAKE_CXX_FLAGS=${CMAKE_CXX_FLAGS}"
-                 "-DCMAKE_Fortran_FLAGS=${CMAKE_Fortran_FLAGS}"
+                 "-DCMAKE_C_FLAGS:STRING=${CMAKE_C_FLAGS}"
+                 "-DCMAKE_CXX_FLAGS:STRING=${CMAKE_CXX_FLAGS}"
+                 "-DCMAKE_Fortran_FLAGS:STRING=${CMAKE_Fortran_FLAGS}"
       OUTPUT_VARIABLE _output)
     file(WRITE "${FortranCInterface_BINARY_DIR}/Verify${lang}/output.txt" "${_output}")
 
diff --git a/Modules/FortranCInterface/Detect.cmake b/Modules/FortranCInterface/Detect.cmake
index 0f964a906..2ea43d132 100644
--- a/Modules/FortranCInterface/Detect.cmake
+++ b/Modules/FortranCInterface/Detect.cmake
@@ -40,8 +40,8 @@ try_compile(FortranCInterface_COMPILED
   ${FortranCInterface_SOURCE_DIR}
   FortranCInterface
   CMAKE_FLAGS
-    "-DCMAKE_C_FLAGS=${CMAKE_C_FLAGS}"
-    "-DCMAKE_Fortran_FLAGS=${CMAKE_Fortran_FLAGS}"
+    "-DCMAKE_C_FLAGS:STRING=${CMAKE_C_FLAGS}"
+    "-DCMAKE_Fortran_FLAGS:STRING=${CMAKE_Fortran_FLAGS}"
   OUTPUT_VARIABLE FortranCInterface_OUTPUT)
 set(FortranCInterface_COMPILED ${FortranCInterface_COMPILED})
 unset(FortranCInterface_COMPILED CACHE)
diff --git a/Tests/FortranC/CMakeLists.txt b/Tests/FortranC/CMakeLists.txt
index e4e8ca1ae..12d416512 100644
--- a/Tests/FortranC/CMakeLists.txt
+++ b/Tests/FortranC/CMakeLists.txt
@@ -13,11 +13,11 @@ file(REMOVE_RECURSE ${CMAKE_BINARY_DIR}/CMakeFiles/FortranCInterface)
 
 if(FortranC_TEST_FLAGS)
   # Test whether FortranCInterface checks see C flags.
-  set(ENV{TEST_OPT_CC} "--test-opt-cc")
+  set(ENV{TEST_OPT_CC} "--test-opt-cc=1")
   set(CMAKE_C_FLAGS "$ENV{TEST_OPT_CC}")
 
   # Test whether FortranCInterface checks see Fortran flags.
-  set(ENV{TEST_OPT_FC} "--test-opt-fc")
+  set(ENV{TEST_OPT_FC} "--test-opt-fc=1")
   set(CMAKE_Fortran_FLAGS "$ENV{TEST_OPT_FC}")
 endif()