diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index e5e78fb..458bb51 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -321,7 +321,7 @@
 /**
  * \brief           Make signature, including padding if relevant.
  *
- * \param ctx       PK context to use
+ * \param ctx       PK context to use - must hold a private key
  * \param md_alg    Hash algorithm used (see notes)
  * \param hash      Hash of the message to sign
  * \param hash_len  Hash length or 0 (see notes)
@@ -350,7 +350,7 @@
 /**
  * \brief           Decrypt message (including padding if relevant).
  *
- * \param ctx       PK context to use
+ * \param ctx       PK context to use - must hold a private key
  * \param input     Input to decrypt
  * \param ilen      Input size
  * \param output    Decrypted output
diff --git a/library/rsa.c b/library/rsa.c
index 3883d09..1f907b7 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -359,6 +359,10 @@
     size_t olen;
     mbedtls_mpi T, T1, T2;
 
+    /* Make sure we have private key info, prevent possible misuse */
+    if( ctx->P.p == NULL || ctx->Q.p == NULL || ctx->D.p == NULL )
+        return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
+
     mbedtls_mpi_init( &T ); mbedtls_mpi_init( &T1 ); mbedtls_mpi_init( &T2 );
 
 #if defined(MBEDTLS_THREADING_C)