diff --git a/sys/src/libmp/port/mpexp.c b/sys/src/libmp/port/mpexp.c
index 1ebabba93..534fb33a9 100644
--- a/sys/src/libmp/port/mpexp.c
+++ b/sys/src/libmp/port/mpexp.c
@@ -22,7 +22,7 @@ mpexp(mpint *b, mpint *e, mpint *m, mpint *res)
 	mpdigit d, bit;
 	int i, j;
 
-	assert(m->flags & MPnorm);
+	assert(m == nil || m->flags & MPnorm);
 	assert((e->flags & MPtimesafe) == 0);
 	res->flags |= b->flags & MPtimesafe;