diff --git a/library/asn1parse.c b/library/asn1parse.c
index f6b79ef..9d42f05 100644
--- a/library/asn1parse.c
+++ b/library/asn1parse.c
@@ -220,7 +220,7 @@
     if( ( ret = asn1_get_tag( p, end, len, ASN1_BIT_STRING ) ) != 0 )
         return( ret );
 
-    if( --*len < 1 || *(*p)++ != 0 )
+    if( (*len)-- < 2 || *(*p)++ != 0 )
         return( POLARSSL_ERR_ASN1_INVALID_DATA );
 
     return( 0 );