diff --git a/library/ecdh.c b/library/ecdh.c
index e30ae3a..df21eac 100644
--- a/library/ecdh.c
+++ b/library/ecdh.c
@@ -212,8 +212,11 @@
                 != 0 )
         return( ret );
 
-    *olen = mpi_size( &ctx->z );
-    return mpi_write_binary( &ctx->z, buf, blen );
+    if( mpi_size( &ctx->z ) > blen )
+        return( POLARSSL_ERR_ECP_BAD_INPUT_DATA );
+
+    *olen = ctx->grp.nbits / 8 + ( ( ctx->grp.nbits % 8 ) != 0 );
+    return mpi_write_binary( &ctx->z, buf, *olen );
 }