diff --git a/radio/src/CMakeLists.txt b/radio/src/CMakeLists.txt
index d223ae2c1..a1836e0c4 100644
--- a/radio/src/CMakeLists.txt
+++ b/radio/src/CMakeLists.txt
@@ -411,7 +411,7 @@ if(NOT MSVC)
   else()
     set(CMAKE_C_COMPILER avr-gcc)
     set(CMAKE_CXX_COMPILER avr-gcc)
-    set(CMAKE_CXX_FLAGS "-mmcu=${MCU} -DF_CPU=16000000UL -O${OPT} -gdwarf-2 -fno-exceptions -flto -fwhole-program -fno-inline-small-functions -mstrict-X ${WARNING_FLAGS} /MP")
+    set(CMAKE_CXX_FLAGS "-mmcu=${MCU} -DF_CPU=16000000UL -O${OPT} -gdwarf-2 -fno-exceptions -flto -fwhole-program -fno-inline-small-functions -mstrict-X ${WARNING_FLAGS}")
 
     # Remove once the minimum cmake version is set to 3.1 in the main CmakeLists.tx
     if (${CMAKE_VERSION} VERSION_LESS 3.1.0})
@@ -463,4 +463,6 @@ if(NOT MSVC)
       WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
       )
   endif()
+else()
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /MP")
 endif(NOT MSVC)