diff --git a/lib/chck b/lib/chck
index 56f2a99..8de0be8 160000
--- a/lib/chck
+++ b/lib/chck
@@ -1 +1 @@
-Subproject commit 56f2a9931ec0daf4ffab89791a2da75791159555
+Subproject commit 8de0be809d20c60ab3e6f41a224db10dd4d399e2
diff --git a/src/pi9.c b/src/pi9.c
index 4218699..ac4a29e 100644
--- a/src/pi9.c
+++ b/src/pi9.c
@@ -110,6 +110,7 @@ static const struct {
 static inline bool
 write_qid(struct pi9_qid *qid, struct pi9_stream *stream)
 {
+   assert(qid && stream);
    return (chck_buffer_write_int(&qid->type, sizeof(qid->type), &stream->out) &&
            chck_buffer_write_int(&qid->vers, sizeof(qid->vers), &stream->out) &&
            chck_buffer_write_int(&qid->path, sizeof(qid->path), &stream->out));
@@ -118,6 +119,7 @@ write_qid(struct pi9_qid *qid, struct pi9_stream *stream)
 static inline bool
 read_qid(struct pi9_qid *qid, struct pi9_stream *stream)
 {
+   assert(qid && stream);
    return (chck_buffer_read_int(&qid->type, sizeof(qid->type), &stream->in) &&
            chck_buffer_read_int(&qid->vers, sizeof(qid->vers), &stream->in) &&
            chck_buffer_read_int(&qid->path, sizeof(qid->path), &stream->in));
@@ -126,6 +128,8 @@ read_qid(struct pi9_qid *qid, struct pi9_stream *stream)
 static inline bool
 read_stat(struct pi9_stat *stat, struct pi9_stream *stream)
 {
+   assert(stat && stream);
+
    uint16_t size;
    if (!chck_buffer_read_int(&size, sizeof(size), &stream->in) ||
        !chck_buffer_read_int(&stat->type, sizeof(stat->type), &stream->in) ||
@@ -248,7 +252,7 @@ op_Tauth(struct pi9 *pi9, uint16_t tag, struct pi9_stream *stream)
    if (!chck_buffer_write_int(&size, sizeof(size), &stream->out) ||
        !chck_buffer_write_int((uint8_t[]){Rauth}, sizeof(uint8_t), &stream->out) ||
        !chck_buffer_write_int(&tag, sizeof(tag), &stream->out) ||
-       !write_qid(qid, stream))
+       (qid && !write_qid(qid, stream)))
       goto err_write;
 
    return true;
diff --git a/src/pi9_string.h b/src/pi9_string.h
index e38075e..2429cae 100644
--- a/src/pi9_string.h
+++ b/src/pi9_string.h
@@ -11,6 +11,8 @@ struct pi9_string {
    bool is_heap;
 };
 
+#define PSTRE(x) (x ? x : "")
+
 static inline bool
 pi9_cstr_is_empty(const char *data)
 {
@@ -21,14 +23,14 @@ static inline bool
 pi9_cstr_ends_with(const char *a, const char *b)
 {
    const size_t lena = (a ? strlen(a) : 0), lenb = (b ? strlen(b) : 0);
-   return (lena >= lenb && !memcmp(a + lena - lenb, b, lenb));
+   return (lena >= lenb && !memcmp(a + lena - lenb, PSTRE(b), lenb));
 }
 
 static inline bool
 pi9_cstr_starts_with(const char *a, const char *b)
 {
    const size_t lena = (a ? strlen(a) : 0), lenb = (b ? strlen(b) : 0);
-   return (lena >= lenb && !memcmp(a, b, lenb));
+   return (lena >= lenb && !memcmp(PSTRE(a), PSTRE(b), lenb));
 }
 
 static inline bool
@@ -53,41 +55,43 @@ static inline bool
 pi9_string_ends_with_cstr(const struct pi9_string *a, const char *cstr)
 {
    const size_t len = (cstr ? strlen(cstr) : 0);
-   return (a->size >= len && !memcmp(a->data + a->size - len, cstr, len));
+   return (a->size >= len && !memcmp(a->data + a->size - len, PSTRE(cstr), len));
 }
 
 static inline bool
 pi9_string_starts_with_cstr(const struct pi9_string *a, const char *cstr)
 {
    const size_t len = (cstr ? strlen(cstr) : 0);
-   return (a->size >= len && !memcmp(a->data, cstr, len));
+   return (a->size >= len && !memcmp(a->data, PSTRE(cstr), len));
 }
 
 static inline bool
 pi9_string_ends_with(const struct pi9_string *a, const struct pi9_string *b)
 {
-   return (a->size >= b->size && !memcmp(a->data + a->size - b->size, b->data, b->size));
+   return (a->size >= b->size && !memcmp(a->data + a->size - b->size, PSTRE(b->data), b->size));
 }
 
 static inline bool
 pi9_string_starts_with(const struct pi9_string *a, const struct pi9_string *b)
 {
-   return (a->size >= b->size && !memcmp(a->data, b->data, b->size));
+   return (a->size >= b->size && !memcmp(PSTRE(a->data), PSTRE(b->data), b->size));
 }
 
 static inline bool
 pi9_string_eq(const struct pi9_string *a, const struct pi9_string *b)
 {
-   return (a->data == b->data) || (a->size == b->size && !memcmp(a->data, b->data, a->size));
+   return (a->data == b->data) || (a->size == b->size && !memcmp(PSTRE(a->data), PSTRE(b->data), a->size));
 }
 
 static inline bool
 pi9_string_eq_cstr(const struct pi9_string *a, const char *cstr)
 {
    const size_t len = (cstr ? strlen(cstr) : 0);
-   return (len == a->size) && (cstr == a->data || !memcmp(a->data, cstr, a->size));
+   return (len == a->size) && (cstr == a->data || !memcmp(PSTRE(a->data), PSTRE(cstr), a->size));
 }
 
+#undef PSTRE
+
 void pi9_string_release(struct pi9_string *string);
 bool pi9_string_set_cstr(struct pi9_string *string, const char *data, bool is_heap);
 bool pi9_string_set_cstr_with_length(struct pi9_string *string, const char *data, size_t len, bool is_heap);