diff --git a/usr.bin/sed/compile.c b/usr.bin/sed/compile.c
index e14666883d06..c7fbe21f9d28 100644
--- a/usr.bin/sed/compile.c
+++ b/usr.bin/sed/compile.c
@@ -558,7 +558,7 @@ compile_flags(char *p, struct s_subst *s)
 {
 	int gn;			/* True if we have seen g or n */
 	unsigned long nval;
-	char wfile[_POSIX2_LINE_MAX + 1], *q;
+	char wfile[_POSIX2_LINE_MAX + 1], *q, *eq;
 
 	s->n = 1;				/* Default */
 	s->p = 0;
@@ -611,9 +611,12 @@ compile_flags(char *p, struct s_subst *s)
 #endif
 			EATSPACE();
 			q = wfile;
+			eq = wfile + sizeof(wfile) - 1;
 			while (*p) {
 				if (*p == '\n')
 					break;
+				if (q >= eq)
+					err(1, "wfile too long");
 				*q++ = *p++;
 			}
 			*q = '\0';