diff --git a/CMakeLists.txt b/CMakeLists.txt
index 112b8a5..61b7a6c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -78,7 +78,7 @@ if(AMALGAMATE_SOURCES)
     string(REPLACE "#include \"${REPLACE_STRING}.h\"" "" AMAL_MINIZ_H "${AMAL_MINIZ_H}")
     string(REPLACE "#include \"${REPLACE_STRING}.h\"" "" AMAL_MINIZ_C "${AMAL_MINIZ_C}")
   endforeach()
-  string(CONCAT AMAL_MINIZ_H "#define MINIZ_EXPORT\n" "${AMAL_MINIZ_H}")
+  string(CONCAT AMAL_MINIZ_H "#ifndef MINIZ_EXPORT\n#define MINIZ_EXPORT\n#endif\n" "${AMAL_MINIZ_H}")
   if(BUILD_HEADER_ONLY)
     string(CONCAT AMAL_MINIZ_H "${AMAL_MINIZ_H}" "\n#ifndef MINIZ_HEADER_FILE_ONLY\n"
              "${AMAL_MINIZ_C}" "\n#endif // MINIZ_HEADER_FILE_ONLY\n")