diff --git a/string.c b/string.c
index b831e9e931200f..33e14c1f24d284 100644
--- a/string.c
+++ b/string.c
@@ -3497,7 +3497,7 @@ str_casecmp(VALUE str1, VALUE str2)
 {
     long len;
     rb_encoding *enc;
-    char *p1, *p1end, *p2, *p2end;
+    const char *p1, *p1end, *p2, *p2end;
 
     enc = rb_enc_compatible(str1, str2);
     if (!enc) {