diff --git a/include/evm.h b/include/evm.h
index 84fee65..f796be7 100644
--- a/include/evm.h
+++ b/include/evm.h
@@ -192,10 +192,14 @@ struct evm_result {
     /// The memory containing the output data is owned by EVM and has to be
     /// freed with evm_result::release().
     ///
+    /// This MAY be NULL.
+    ///
     /// @todo Inconsistent name: output_data vs msg.input.
     uint8_t const* output_data;
 
     /// The size of the output data.
+    ///
+    /// If output_data is NULL this MUST be 0.
     size_t output_size;
 
     /// The pointer to a function releasing all resources associated with