diff --git a/library/rsa.c b/library/rsa.c
index 6b720a4..923294f 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -1376,6 +1376,9 @@
     //
     msb = mpi_msb( &ctx->N ) - 1;
 
+    if( buf[0] >> ( 8 - siglen * 8 + msb ) )
+        return( POLARSSL_ERR_RSA_BAD_INPUT_DATA );
+
     // Compensate for boundary condition when applying mask
     //
     if( msb % 8 == 0 )
@@ -1383,9 +1386,6 @@
         p++;
         siglen -= 1;
     }
-    else
-    if( buf[0] >> ( 8 - siglen * 8 + msb ) )
-        return( POLARSSL_ERR_RSA_BAD_INPUT_DATA );
 
     if( siglen < hlen + 2 )
         return( POLARSSL_ERR_RSA_BAD_INPUT_DATA );