--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -173,12 +173,13 @@
 
 if (NOT EXTERNAL_FMT)
     set(FMT_INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/contrib/fmt/include")
+    include_directories(${FMT_INCLUDE_DIR})
 endif()
 
 include_directories(${CMAKE_CURRENT_SOURCE_DIR}/src ${CMAKE_CURRENT_BINARY_DIR})
 
 find_package(Osmium 2.17.3 REQUIRED COMPONENTS io)
-include_directories(SYSTEM ${OSMIUM_INCLUDE_DIRS} ${PROTOZERO_INCLUDE_DIR} ${FMT_INCLUDE_DIR})
+include_directories(SYSTEM ${OSMIUM_INCLUDE_DIRS} ${PROTOZERO_INCLUDE_DIR})
 
 if (WITH_LUA)
     if (WITH_LUAJIT)