diff --git a/CMakeLists.txt b/CMakeLists.txt
index cc58e40efed6..e18f607be84e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -222,7 +222,7 @@ if(CMAKE_COMPILER_IS_CLANG)
 endif(CMAKE_COMPILER_IS_CLANG)
 
 if(CMAKE_COMPILER_IS_IAR)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --warn_about_c_style_casts --warnings_are_errors -Ohz")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --warn_about_c_style_casts -Ohz")
 endif(CMAKE_COMPILER_IS_IAR)
 
 if(CMAKE_COMPILER_IS_MSVC)
@@ -243,6 +243,10 @@ if(MBEDTLS_FATAL_WARNINGS)
             set(CMAKE_C_FLAGS_ASANDBG "${CMAKE_C_FLAGS_ASANDBG} -Wno-error=cpp")
         endif(UNSAFE_BUILD)
     endif(CMAKE_COMPILER_IS_CLANG OR CMAKE_COMPILER_IS_GNU)
+
+    if (CMAKE_COMPILER_IS_IAR)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --warning_are_errors")
+    endif(CMAKE_COMPILER_IS_IAR)
 endif(MBEDTLS_FATAL_WARNINGS)
 
 if(CMAKE_BUILD_TYPE STREQUAL "Coverage")
diff --git a/ChangeLog.d/fix_cmake_using_iar_toolchain.txt b/ChangeLog.d/fix_cmake_using_iar_toolchain.txt
new file mode 100644
index 000000000000..ecc09c241e3f
--- /dev/null
+++ b/ChangeLog.d/fix_cmake_using_iar_toolchain.txt
@@ -0,0 +1,3 @@
+Bugfix
+   * Fixed an issue that cause compile error using CMake IAR toolchain.
+     Fixes #5964.