diff --git a/include/prism/internal/memchr.h b/include/prism/internal/memchr.h
index 63c738387d..6f6b0bca30 100644
--- a/include/prism/internal/memchr.h
+++ b/include/prism/internal/memchr.h
@@ -10,6 +10,6 @@
  * we need to search for a character in a buffer that could be the trailing byte
  * of a multibyte character.
  */
-void * pm_memchr(const void *source, int character, size_t number, bool encoding_changed, const pm_encoding_t *encoding);
+const void * pm_memchr(const void *source, int character, size_t number, bool encoding_changed, const pm_encoding_t *encoding);
 
 #endif
diff --git a/src/memchr.c b/src/memchr.c
index 6266d4ca7a..900e6245b7 100644
--- a/src/memchr.c
+++ b/src/memchr.c
@@ -11,7 +11,7 @@
  * we need to search for a character in a buffer that could be the trailing byte
  * of a multibyte character.
  */
-void *
+const void *
 pm_memchr(const void *memory, int character, size_t number, bool encoding_changed, const pm_encoding_t *encoding) {
     if (encoding_changed && encoding->multibyte && character >= TRAILING_BYTE_MINIMUM) {
         const uint8_t *source = (const uint8_t *) memory;