diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index e2529e4..310aeef 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -530,9 +530,13 @@
  * \brief           Parse a private key in PEM or DER format
  *
  * \param ctx       key to be initialized
- * \param key       input buffer
- * \param keylen    size of the buffer
- *                  (including the terminating null byte for PEM data)
+ * \param key       Input buffer to parse.
+ *                  The buffer must contain the input exactly, with no
+ *                  extra trailing material. For PEM, the buffer must
+ *                  contain a null-terminated string.
+ * \param keylen    Size of \b key in bytes.
+ *                  For PEM data, this includes the terminating null byte,
+ *                  so \p keylen must be equal to `strlen(key) + 1`.
  * \param pwd       password for decryption (optional)
  * \param pwdlen    size of the password
  *
@@ -553,9 +557,13 @@
  * \brief           Parse a public key in PEM or DER format
  *
  * \param ctx       key to be initialized
- * \param key       input buffer
- * \param keylen    size of the buffer
- *                  (including the terminating null byte for PEM data)
+ * \param key       Input buffer to parse.
+ *                  The buffer must contain the input exactly, with no
+ *                  extra trailing material. For PEM, the buffer must
+ *                  contain a null-terminated string.
+ * \param keylen    Size of \b key in bytes.
+ *                  For PEM data, this includes the terminating null byte,
+ *                  so \p keylen must be equal to `strlen(key) + 1`.
  *
  * \note            On entry, ctx must be empty, either freshly initialised
  *                  with mbedtls_pk_init() or reset with mbedtls_pk_free(). If you need a
@@ -643,9 +651,10 @@
 /**
  * \brief           Write a public key to a PEM string
  *
- * \param ctx       public key to write away
- * \param buf       buffer to write to
- * \param size      size of the buffer
+ * \param ctx       Context containing the public key to write.
+ * \param buf       Buffer to write to. The output includes a
+ *                  terminating null byte.
+ * \param size      Size of the buffer in bytes.
  *
  * \return          0 if successful, or a specific error code
  */
@@ -654,9 +663,10 @@
 /**
  * \brief           Write a private key to a PKCS#1 or SEC1 PEM string
  *
- * \param ctx       private to write away
- * \param buf       buffer to write to
- * \param size      size of the buffer
+ * \param ctx       Context containing the private key to write.
+ * \param buf       Buffer to write to. The output includes a
+ *                  terminating null byte.
+ * \param size      Size of the buffer in bytes.
  *
  * \return          0 if successful, or a specific error code
  */
diff --git a/library/pkparse.c b/library/pkparse.c
index 7c14e34..127f9b8 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -1170,15 +1170,16 @@
 #endif
 
     PK_VALIDATE_RET( pk != NULL );
-    PK_VALIDATE_RET( key != NULL || keylen == 0 );
-    PK_VALIDATE_RET( pwd != NULL || pwdlen == 0 );
+    if( keylen == 0 )
+        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT );
+    PK_VALIDATE_RET( key != NULL );
 
 #if defined(MBEDTLS_PEM_PARSE_C)
    mbedtls_pem_init( &pem );
 
 #if defined(MBEDTLS_RSA_C)
     /* Avoid calling mbedtls_pem_read_buffer() on non-null-terminated string */
-    if( keylen == 0 || key[keylen - 1] != '\0' )
+    if( key[keylen - 1] != '\0' )
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     else
         ret = mbedtls_pem_read_buffer( &pem,
@@ -1209,7 +1210,7 @@
 
 #if defined(MBEDTLS_ECP_C)
     /* Avoid calling mbedtls_pem_read_buffer() on non-null-terminated string */
-    if( keylen == 0 || key[keylen - 1] != '\0' )
+    if( key[keylen - 1] != '\0' )
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     else
         ret = mbedtls_pem_read_buffer( &pem,
@@ -1239,7 +1240,7 @@
 #endif /* MBEDTLS_ECP_C */
 
     /* Avoid calling mbedtls_pem_read_buffer() on non-null-terminated string */
-    if( keylen == 0 || key[keylen - 1] != '\0' )
+    if( key[keylen - 1] != '\0' )
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     else
         ret = mbedtls_pem_read_buffer( &pem,
@@ -1262,7 +1263,7 @@
 
 #if defined(MBEDTLS_PKCS12_C) || defined(MBEDTLS_PKCS5_C)
     /* Avoid calling mbedtls_pem_read_buffer() on non-null-terminated string */
-    if( keylen == 0 || key[keylen - 1] != '\0' )
+    if( key[keylen - 1] != '\0' )
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     else
         ret = mbedtls_pem_read_buffer( &pem,
@@ -1300,9 +1301,6 @@
     {
         unsigned char *key_copy;
 
-        if( keylen == 0 )
-            return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT );
-
         if( ( key_copy = mbedtls_calloc( 1, keylen ) ) == NULL )
             return( MBEDTLS_ERR_PK_ALLOC_FAILED );
 
@@ -1387,13 +1385,15 @@
 #endif
 
     PK_VALIDATE_RET( ctx != NULL );
+    if( keylen == 0 )
+        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT );
     PK_VALIDATE_RET( key != NULL || keylen == 0 );
 
 #if defined(MBEDTLS_PEM_PARSE_C)
     mbedtls_pem_init( &pem );
 #if defined(MBEDTLS_RSA_C)
     /* Avoid calling mbedtls_pem_read_buffer() on non-null-terminated string */
-    if( keylen == 0 || key[keylen - 1] != '\0' )
+    if( key[keylen - 1] != '\0' )
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     else
         ret = mbedtls_pem_read_buffer( &pem,
@@ -1424,7 +1424,7 @@
 #endif /* MBEDTLS_RSA_C */
 
     /* Avoid calling mbedtls_pem_read_buffer() on non-null-terminated string */
-    if( keylen == 0 || key[keylen - 1] != '\0' )
+    if( key[keylen - 1] != '\0' )
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     else
         ret = mbedtls_pem_read_buffer( &pem,
diff --git a/library/pkwrite.c b/library/pkwrite.c
index 51d0c56..8d1da2f 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -186,7 +186,9 @@
     const char *oid;
 
     PK_VALIDATE_RET( key != NULL );
-    PK_VALIDATE_RET( buf != NULL || size == 0 );
+    if( size == 0 )
+        return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+    PK_VALIDATE_RET( buf != NULL );
 
     c = buf + size;
 
@@ -236,7 +238,9 @@
     size_t len = 0;
 
     PK_VALIDATE_RET( key != NULL );
-    PK_VALIDATE_RET( buf != NULL || size == 0 );
+    if( size == 0 )
+        return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+    PK_VALIDATE_RET( buf != NULL );
 
     c = buf + size;