diff --git a/CMakeLists.txt b/CMakeLists.txt
index 54d4817..300a6a5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -156,7 +156,9 @@ endif()
 if(NOT DEACTIVATE_ZLIB)
     # import the ZLIB_ROOT environment variable to help finding the zlib library
     if(PREFER_EXTERNAL_ZLIB)
-        set(ZLIB_ROOT $ENV{ZLIB_ROOT})
+        if(NOT DEFINED ZLIB_ROOT)
+            set(ZLIB_ROOT $ENV{ZLIB_ROOT})
+        endif()
         find_package(ZLIB)
         if(NOT ZLIB_FOUND )
             message(STATUS "No zlib found.  Using internal sources.")