diff --git a/src/pricehist/sources/alphavantage.py b/src/pricehist/sources/alphavantage.py
index 07aaae2..d0eac8c 100644
--- a/src/pricehist/sources/alphavantage.py
+++ b/src/pricehist/sources/alphavantage.py
@@ -51,6 +51,9 @@ class AlphaVantage(BaseSource):
             "The PAIR for stocks is the stock symbol only. The quote currency "
             f"will be determined automatically. {self._stock_symbols_message()}\n"
             "The price type 'adjclose' is only available for stocks.\n"
+            "Beware that digital currencies quoted in non-USD currencies may "
+            "be converted from USD data at one recent exchange rate rather "
+            "than using historical rates.\n"
             "Alpha Vantage's standard API call frequency limits is 5 calls per "
             "minute and 500 per day, so you may need to pause between successive "
             "commands. Note that retrieving prices for one stock requires two "