diff --git a/src/commands.c b/src/commands.c
index 368a495..ba42a9b 100644
--- a/src/commands.c
+++ b/src/commands.c
@@ -1,5 +1,5 @@
-#ifndef _DEFAULT_SOURCE
-#define _DEFAULT_SOURCE  /* for strcasestr() on glibc */
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE      /* for strcasestr() on glibc and musl */
 #endif
 #if defined(__APPLE__) && !defined(_DARWIN_C_SOURCE)
 #define _DARWIN_C_SOURCE /* for strcasestr() on macOS */
diff --git a/src/message.c b/src/message.c
index b37b9a2..098df5b 100644
--- a/src/message.c
+++ b/src/message.c
@@ -1,5 +1,5 @@
-#ifndef _DEFAULT_SOURCE
-#define _DEFAULT_SOURCE  /* for strcasestr() on glibc */
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE      /* for strcasestr() on glibc and musl */
 #endif
 #if defined(__APPLE__) && !defined(_DARWIN_C_SOURCE)
 #define _DARWIN_C_SOURCE /* for strcasestr() on macOS */