diff --git a/docs/units-and-global-variables.rst b/docs/units-and-global-variables.rst
index 1cad59e42..dc6cefe91 100644
--- a/docs/units-and-global-variables.rst
+++ b/docs/units-and-global-variables.rst
@@ -68,7 +68,7 @@ or are general-use utility functions.
 Block and Transaction Properties
 --------------------------------
 
-- ``blockhash(uint blockNumber) returns (bytes32)``: hash of the given block - only works for 256 most recent, excluding current, blocks
+- ``blockhash(uint blockNumber) returns (bytes32)``: hash of the given block when ``blocknumber`` is one of the 256 most recent blocks; otherwise returns zero
 - ``block.chainid`` (``uint``): current chain id
 - ``block.coinbase`` (``address payable``): current block miner's address
 - ``block.difficulty`` (``uint``): current block difficulty