diff --git a/library/pkwrite.c b/library/pkwrite.c
index 35dbd0b..bb9514e 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -97,7 +97,7 @@
         return( ret );
     }
 
-    if( *p - start < (int) len )
+    if( *p < start || (size_t)( *p - start ) < len )
         return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
 
     *p -= len;