diff --git a/include/mbedtls/pk_internal.h b/include/mbedtls/pk_internal.h
index d3b501d..0fda01d 100644
--- a/include/mbedtls/pk_internal.h
+++ b/include/mbedtls/pk_internal.h
@@ -41,18 +41,19 @@
     /** Type name */
     const char *name;
 
-    /** Get key size in bits */
+    /** Get key size in bits (must be valid)*/
     size_t (*get_bitlen)( const void * );
 
-    /** Tell if the context implements this type (e.g. ECKEY can do ECDSA) */
+    /** Tell if the context implements this type (e.g. ECKEY can do ECDSA)
+     * (must be valid) */
     int (*can_do)( mbedtls_pk_type_t type );
 
-    /** Verify signature */
+    /** Verify signature (may be NULL) */
     int (*verify_func)( void *ctx, mbedtls_md_type_t md_alg,
                         const unsigned char *hash, size_t hash_len,
                         const unsigned char *sig, size_t sig_len );
 
-    /** Make signature */
+    /** Make signature (may be NULL)*/
     int (*sign_func)( void *ctx, mbedtls_md_type_t md_alg,
                       const unsigned char *hash, size_t hash_len,
                       unsigned char *sig, size_t *sig_len,
@@ -60,13 +61,13 @@
                       void *p_rng );
 
 #if defined(MBEDTLS_ECDSA_C) && defined(MBEDTLS_ECP_RESTARTABLE)
-    /** Verify signature (restartable) */
+    /** Verify signature (restartable) (may be NULL) */
     int (*verify_rs_func)( void *ctx, mbedtls_md_type_t md_alg,
                            const unsigned char *hash, size_t hash_len,
                            const unsigned char *sig, size_t sig_len,
                            void *rs_ctx );
 
-    /** Make signature (restartable) */
+    /** Make signature (restartable) (may be NULL) */
     int (*sign_rs_func)( void *ctx, mbedtls_md_type_t md_alg,
                          const unsigned char *hash, size_t hash_len,
                          unsigned char *sig, size_t *sig_len,
@@ -74,36 +75,36 @@
                          void *p_rng, void *rs_ctx );
 #endif /* MBEDTLS_ECDSA_C && MBEDTLS_ECP_RESTARTABLE */
 
-    /** Decrypt message */
+    /** Decrypt message (may be NULL) */
     int (*decrypt_func)( void *ctx, const unsigned char *input, size_t ilen,
                          unsigned char *output, size_t *olen, size_t osize,
                          int (*f_rng)(void *, unsigned char *, size_t),
                          void *p_rng );
 
-    /** Encrypt message */
+    /** Encrypt message (may be NULL ) */
     int (*encrypt_func)( void *ctx, const unsigned char *input, size_t ilen,
                          unsigned char *output, size_t *olen, size_t osize,
                          int (*f_rng)(void *, unsigned char *, size_t),
                          void *p_rng );
 
-    /** Check public-private key pair */
+    /** Check public-private key pair (may be NULL) */
     int (*check_pair_func)( const void *pub, const void *prv );
 
-    /** Allocate a new context */
+    /** Allocate a new context (must be valid) */
     void * (*ctx_alloc_func)( void );
 
-    /** Free the given context */
+    /** Free the given context (must be valid) */
     void (*ctx_free_func)( void *ctx );
 
 #if defined(MBEDTLS_ECDSA_C) && defined(MBEDTLS_ECP_RESTARTABLE)
-    /** Allocate the restart context */
+    /** Allocate the restart context (may be NULL)*/
     void * (*rs_alloc_func)( void );
 
-    /** Free the restart context */
+    /** Free the restart context (may be NULL) */
     void (*rs_free_func)( void *rs_ctx );
 #endif /* MBEDTLS_ECDSA_C && MBEDTLS_ECP_RESTARTABLE */
 
-    /** Interface with the debug module */
+    /** Interface with the debug module (may be NULL) */
     void (*debug_func)( const void *ctx, mbedtls_pk_debug_item *items );
 
 };
diff --git a/library/pk.c b/library/pk.c
index 2519d87..9ded7e0 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -92,6 +92,9 @@
     const unsigned char *hash, size_t hash_len,
     const unsigned char *sig, size_t sig_len )
 {
+    if( info->verify_func == NULL )
+        return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
+
     return( info->verify_func( ctx, md_alg, hash, hash_len, sig, sig_len ) );
 }
 
@@ -102,6 +105,9 @@
     int (*f_rng)(void *, unsigned char *, size_t),
     void *p_rng )
 {
+    if( info->sign_func == NULL )
+        return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
+
     return( info->sign_func( ctx, md_alg, hash, hash_len, sig, sig_len,
                              f_rng, p_rng ) );
 }
@@ -113,6 +119,9 @@
     int (*f_rng)(void *, unsigned char *, size_t),
     void *p_rng )
 {
+    if( info->decrypt_func == NULL )
+        return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
+
     return( info->decrypt_func( ctx, input, ilen, output, olen, osize,
                                 f_rng, p_rng ) );
 }
@@ -124,6 +133,9 @@
     int (*f_rng)(void *, unsigned char *, size_t),
     void *p_rng )
 {
+    if( info->encrypt_func == NULL )
+        return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
+
     return( info->encrypt_func( ctx, input, ilen, output, olen, osize,
                                 f_rng, p_rng ) );
 }
@@ -131,6 +143,9 @@
 MBEDTLS_ALWAYS_INLINE static inline int pk_info_check_pair_func(
     const mbedtls_pk_info_t *info, const void *pub, const void *prv )
 {
+    if( info->check_pair_func == NULL )
+        return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );
+
     return( info->check_pair_func( pub, prv ) );
 }
 
@@ -146,11 +161,15 @@
     info->ctx_free_func( ctx );
 }
 
-MBEDTLS_ALWAYS_INLINE static inline void pk_info_debug_func(
+MBEDTLS_ALWAYS_INLINE static inline int pk_info_debug_func(
     const mbedtls_pk_info_t *info,
     const void *ctx, mbedtls_pk_debug_item *items )
 {
+    if( info->debug_func == NULL )
+        return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
+
     info->debug_func( ctx, items );
+    return( 0 );
 }
 
 /*
@@ -388,9 +407,6 @@
     (void) rs_ctx;
 #endif /* MBEDTLS_ECDSA_C && MBEDTLS_ECP_RESTARTABLE */
 
-    if( ctx->pk_info->verify_func == NULL )
-        return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
-
     return( pk_info_verify_func( ctx->pk_info, ctx->pk_ctx, md_alg, hash, hash_len,
                                        sig, sig_len ) );
 }
@@ -511,9 +527,6 @@
     (void) rs_ctx;
 #endif /* MBEDTLS_ECDSA_C && MBEDTLS_ECP_RESTARTABLE */
 
-    if( ctx->pk_info->sign_func == NULL )
-        return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
-
     return( pk_info_sign_func( ctx->pk_info, ctx->pk_ctx, md_alg, hash, hash_len,
                                      sig, sig_len, f_rng, p_rng ) );
 }
@@ -546,9 +559,6 @@
     if( ctx->pk_info == NULL )
         return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );
 
-    if( ctx->pk_info->decrypt_func == NULL )
-        return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
-
     return( pk_info_decrypt_func( ctx->pk_info, ctx->pk_ctx, input, ilen,
                 output, olen, osize, f_rng, p_rng ) );
 }
@@ -569,9 +579,6 @@
     if( ctx->pk_info == NULL )
         return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );
 
-    if( ctx->pk_info->encrypt_func == NULL )
-        return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
-
     return( pk_info_encrypt_func( ctx->pk_info, ctx->pk_ctx, input, ilen,
                 output, olen, osize, f_rng, p_rng ) );
 }
@@ -600,9 +607,6 @@
             return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
     }
 
-    if( prv->pk_info->check_pair_func == NULL )
-        return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );
-
     return( pk_info_check_pair_func( prv->pk_info, pub->pk_ctx, prv->pk_ctx ) );
 }
 
@@ -628,11 +632,7 @@
     if( ctx->pk_info == NULL )
         return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );
 
-    if( ctx->pk_info->debug_func == NULL )
-        return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
-
-    pk_info_debug_func( ctx->pk_info, ctx->pk_ctx, items );
-    return( 0 );
+    return( pk_info_debug_func( ctx->pk_info, ctx->pk_ctx, items ) );
 }
 
 /*