diff --git a/library/bignum.c b/library/bignum.c
index 7484900..208d508 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -512,6 +512,12 @@
 
     mbedtls_mpi_init( &T );
 
+    if( s[0] == 0 )
+    {
+        mbedtls_mpi_free( X );
+        return( 0 );
+    }
+
     if( s[0] == '-' )
     {
         ++s;