diff --git a/library/rsa.c b/library/rsa.c
index 3bbf24843..119431d8f 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -708,6 +708,12 @@ int mbedtls_rsa_rsaes_oaep_decrypt( mbedtls_rsa_context *ctx,
     if( md_info == NULL )
         return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
 
+    hlen = mbedtls_md_get_size( md_info );
+
+    // checking for integer underflow
+    if( 2 * hlen + 2 > ilen )
+        return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
+
     /*
      * RSA operation
      */
@@ -721,12 +727,6 @@ int mbedtls_rsa_rsaes_oaep_decrypt( mbedtls_rsa_context *ctx,
     /*
      * Unmask data and generate lHash
      */
-    hlen = mbedtls_md_get_size( md_info );
-
-    // checking for integer underflow
-    if( 2 * hlen + 2 > ilen )
-        return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
-
     mbedtls_md_init( &md_ctx );
     mbedtls_md_setup( &md_ctx, md_info, 0 );