diff --git a/src/regexp.c b/src/regexp.c
index 1a5cfd0..bec0464 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -1641,7 +1641,11 @@
 	{
 	    if (enc_utf8 && c > 0x80)
 	    {
-		if (utf_fold(utf_ptr2char(p)) == cc)
+		int uc = utf_ptr2char(p);
+
+		// Do not match an illegal byte.  E.g. 0xff matches 0xc3 0xbf,
+		// not 0xff.
+		if ((uc < 0x80 || uc != *p) && utf_fold(uc) == cc)
 		    return p;
 	    }
 	    else if (*p == c || *p == cc)