diff --git a/Modules/FindPNG.cmake b/Modules/FindPNG.cmake
index 4fb218b..d69e27a 100644
--- a/Modules/FindPNG.cmake
+++ b/Modules/FindPNG.cmake
@@ -39,6 +39,7 @@ if(ZLIB_FOUND)
   if (PNG_LIBRARY AND PNG_PNG_INCLUDE_DIR)
       # png.h includes zlib.h. Sigh.
       SET(PNG_INCLUDE_DIR ${PNG_PNG_INCLUDE_DIR} ${ZLIB_INCLUDE_DIR} )
+      SET(PNG_INCLUDE_DIRS ${PNG_INCLUDE_DIR})
       SET(PNG_LIBRARIES ${PNG_LIBRARY} ${ZLIB_LIBRARY})
 
       if (CYGWIN)