diff --git a/hledger/Hledger/Cli/Commands/Roi.hs b/hledger/Hledger/Cli/Commands/Roi.hs
index b1f278ad6..9bee351d9 100644
--- a/hledger/Hledger/Cli/Commands/Roi.hs
+++ b/hledger/Hledger/Cli/Commands/Roi.hs
@@ -263,6 +263,6 @@ total trans query = unMix $ sumPostings $  filter (matchesPosting query) $ conca
 
 unMix :: MixedAmount -> Quantity
 unMix a =
-  case (normaliseMixedAmount $ mixedAmountCost a) of
-    (Mixed [a]) -> aquantity a
-    _ -> error' "MixedAmount failed to normalize"  -- PARTIAL:
+  case (unifyMixedAmount $ mixedAmountCost a) of
+    Just a -> aquantity a
+    Nothing -> error' $ "Amounts could not be converted to a single cost basis: " ++ show (map showAmount $ amounts a)
diff --git a/hledger/test/roi.test b/hledger/test/roi.test
index 9b9c7242f..0fd8e4ef2 100644
--- a/hledger/test/roi.test
+++ b/hledger/test/roi.test
@@ -225,3 +225,18 @@ hledger -f- roi -p 2019-11
 +---++------------+------------++---------------+----------+-------------+-----++-------+-------+
 
 >>>=0
+
+# 9. Fail with a nice error message when commodity can't be valued
+hledger -f- roi -p 2019-11 --inv Investment --pnl PnL
+<<<
+2019/11/01 Example
+  Assets:Checking  -1 A
+  Investment       10 B
+
+2019/11/02 Example
+  Investment        -10 B @@ 1 A
+  Assets:Checking    12 A
+  Unrealized PnL
+>>>2
+hledger: Amounts could not be converted to a single cost basis: ["10 B","-10 B @@ 1 A"]
+>>>=1