diff --git a/source/mijin/io/stream.cpp b/source/mijin/io/stream.cpp
index 2124edb..4ee29b7 100644
--- a/source/mijin/io/stream.cpp
+++ b/source/mijin/io/stream.cpp
@@ -374,7 +374,7 @@ StreamError FileStream::open(const char* path, FileOpenMode mode_)
 void FileStream::close()
 {
     MIJIN_ASSERT(handle != nullptr, "FileStream is not open.");
-    const int result = std::fclose(handle); // NOLINT(cppcoreguidelines-owning-memory)
+    [[maybe_unused]] const int result = std::fclose(handle); // NOLINT(cppcoreguidelines-owning-memory)
     MIJIN_ASSERT(result == 0, "fclose failed.");
     handle = nullptr;
 }
@@ -453,7 +453,7 @@ StreamError FileStream::seek(std::intptr_t pos, SeekMode seekMode)
 
 void FileStream::flush()
 {
-    const int result = std::fflush(handle);
+    [[maybe_unused]] const int result = std::fflush(handle);
     MIJIN_ASSERT(result == 0, "fflush failed.");
 }
 
@@ -465,7 +465,7 @@ bool FileStream::isAtEnd()
     if (std::feof(handle)) {
         return true;
     }
-    const int result = std::fseek(handle, -1, SEEK_CUR);
+    [[maybe_unused]] const int result = std::fseek(handle, -1, SEEK_CUR);
     MIJIN_ASSERT(result == 0, "fseek failed.");
     return false;
 }