diff --git a/lib/nghttp3_ringbuf.c b/lib/nghttp3_ringbuf.c
index 7d3ab39..85c2e03 100644
--- a/lib/nghttp3_ringbuf.c
+++ b/lib/nghttp3_ringbuf.c
@@ -33,18 +33,20 @@
 
 #include "nghttp3_macro.h"
 
+#ifndef NDEBUG
 static int ispow2(size_t n) {
-#if defined(_MSC_VER) && !defined(__clang__) &&                                \
-  (defined(_M_ARM) || (defined(_M_ARM64) && _MSC_VER < 1941))
+#  if defined(_MSC_VER) && !defined(__clang__) &&                              \
+    (defined(_M_ARM) || (defined(_M_ARM64) && _MSC_VER < 1941))
   return n && !(n & (n - 1));
-#elif defined(WIN32)
+#  elif defined(WIN32)
   return 1 == __popcnt((unsigned int)n);
-#else  /* !((defined(_MSC_VER) && !defined(__clang__) && (defined(_M_ARM) ||   \
-          (defined(_M_ARM64) && _MSC_VER < 1941))) || defined(WIN32)) */
+#  else  /* !((defined(_MSC_VER) && !defined(__clang__) && (defined(_M_ARM) || \
+            (defined(_M_ARM64) && _MSC_VER < 1941))) || defined(WIN32)) */
   return 1 == __builtin_popcount((unsigned int)n);
-#endif /* !((defined(_MSC_VER) && !defined(__clang__) && (defined(_M_ARM) ||   \
-          (defined(_M_ARM64) && _MSC_VER < 1941))) || defined(WIN32)) */
+#  endif /* !((defined(_MSC_VER) && !defined(__clang__) && (defined(_M_ARM) || \
+            (defined(_M_ARM64) && _MSC_VER < 1941))) || defined(WIN32)) */
 }
+#endif /* !defined(NDEBUG) */
 
 int nghttp3_ringbuf_init(nghttp3_ringbuf *rb, size_t nmemb, size_t size,
                          const nghttp3_mem *mem) {