diff --git a/cmake/FindHelper.cmake b/cmake/FindHelper.cmake
index b84b0a2a4..457a7a371 100644
--- a/cmake/FindHelper.cmake
+++ b/cmake/FindHelper.cmake
@@ -40,12 +40,12 @@ macro(FIND_HELPER prefix pkg_name header lib)
             ENV ${prefix}_PATH
             )
         include(FindPackageHandleStandardArgs)
-        if (NOT "${prefix}_INCLUDE_DIRS")
-          set(${prefix}_INCLUDE_DIRS "-I/usr/include")
+        if ("${${prefix}_INCLUDE_DIRS}" STREQUAL "")
+          set(${prefix}_INCLUDE_DIRS "/usr/include")
         endif ()
         FIND_PACKAGE_HANDLE_STANDARD_ARGS(${prefix} DEFAULT_MSG ${prefix}_LIBRARIES ${prefix}_INCLUDE_DIRS)
         if(NOT "${PC_${prefix}_INCLUDE_DIRS}" STREQUAL "")
-            SET(${prefix}_INCLUDE_DIRS "${${prefix}_INCLUDE_DIRS};${PC_${prefix}_INCLUDE_DIRS}")
+            set(${prefix}_INCLUDE_DIRS "${${prefix}_INCLUDE_DIRS};${PC_${prefix}_INCLUDE_DIRS}")
         endif()
     endif()
 endmacro()