diff --git a/ShellCheck/AST.hs b/ShellCheck/AST.hs
index 91337b5..8182b61 100644
--- a/ShellCheck/AST.hs
+++ b/ShellCheck/AST.hs
@@ -34,6 +34,7 @@ data CaseType = CaseBreak | CaseFallThrough | CaseContinue deriving (Show, Eq)
 data Token =
     TA_Binary Id String Token Token
     | TA_Expansion Id [Token]
+    | TA_Index Id Token
     | TA_Sequence Id [Token]
     | TA_Trinary Id Token Token Token
     | TA_Unary Id String Token
@@ -245,6 +246,7 @@ analyze f g i =
         c <- round t3
         return $ TA_Trinary id a b c
     delve (TA_Expansion id t) = dl t $ TA_Expansion id
+    delve (TA_Index id t) = d1 t $ TA_Index id
     delve (T_Annotation id anns t) = d1 t $ T_Annotation id anns
     delve t = return t
 
@@ -330,6 +332,7 @@ getId t = case t of
         TA_Sequence id _  -> id
         TA_Trinary id _ _ _  -> id
         TA_Expansion id _  -> id
+        TA_Index id _  -> id
         T_ProcSub id _ _ -> id
         T_Glob id _ -> id
         T_ForArithmetic id _ _ _ _ -> id
diff --git a/ShellCheck/Analytics.hs b/ShellCheck/Analytics.hs
index 5878906..b8942b3 100644
--- a/ShellCheck/Analytics.hs
+++ b/ShellCheck/Analytics.hs
@@ -1205,19 +1205,28 @@ prop_checkArithmeticDeref5 = verifyNot checkArithmeticDeref "(($1))"
 prop_checkArithmeticDeref6 = verify checkArithmeticDeref "(( a[$i] ))"
 prop_checkArithmeticDeref7 = verifyNot checkArithmeticDeref "(( 10#$n ))"
 prop_checkArithmeticDeref8 = verifyNot checkArithmeticDeref "let i=$i+1"
+prop_checkArithmeticDeref9 = verifyNot checkArithmeticDeref "(( a[foo] ))"
+prop_checkArithmeticDeref10= verifyNot checkArithmeticDeref "(( a[\\$foo] ))"
+prop_checkArithmeticDeref11= verifyNot checkArithmeticDeref "a[$foo]=wee"
+prop_checkArithmeticDeref12= verify checkArithmeticDeref "for ((i=0; $i < 3; i)); do true; done"
 checkArithmeticDeref params t@(TA_Expansion _ [T_DollarBraced id l]) =
-    unless ((isException $ bracedString l) || (not isNormal)) $
-        style id 2004 "$ on variables in (( )) is unnecessary."
+    unless (isException $ bracedString l) getWarning
   where
     isException [] = True
     isException s = any (`elem` "/.:#%?*@") s || isDigit (head s)
-    isNormal = fromMaybe True $ msum $ map isNormalContext $ (parents params t)
-    isNormalContext t =
+    getWarning = fromMaybe noWarning . msum . map warningFor $ parents params t
+    warningFor t =
         case t of
-            T_Arithmetic {} -> return True
-            T_DollarArithmetic {} -> return True
-            T_SimpleCommand {} -> return False
-            _ -> fail "Irrelevant"
+            T_Arithmetic {} -> return normalWarning
+            T_DollarArithmetic {} -> return normalWarning
+            T_ForArithmetic {} -> return normalWarning
+            TA_Index {} -> return indexWarning
+            T_SimpleCommand {} -> return noWarning
+            _ -> Nothing
+
+    normalWarning = style id 2004 "$/${} is unnecessary on arithmetic variables."
+    indexWarning = style id 2149 "Remove $/${} for numeric index, or escape it for string."
+    noWarning = return ()
 checkArithmeticDeref _ _ = return ()
 
 prop_checkArithmeticBadOctal1 = verify checkArithmeticBadOctal "(( 0192 ))"
diff --git a/ShellCheck/Parser.hs b/ShellCheck/Parser.hs
index b0a021e..eec9f3f 100644
--- a/ShellCheck/Parser.hs
+++ b/ShellCheck/Parser.hs
@@ -489,10 +489,10 @@ readArithmeticContents =
 
     readArrayIndex = do
         id <- getNextId
-        start <- literal "["
+        char '['
         middle <- readArithmeticContents
-        end <- literal "]"
-        return $ T_NormalWord id [start, middle, end]
+        char ']'
+        return $ TA_Index id middle
 
     literal s = do
         id <- getNextId
@@ -596,7 +596,7 @@ readArithmeticContents =
             id <- getNextId
             op <- try $ string "++" <|> string "--"
             spacing
-            return $ TA_Unary id ("|" ++ op) x
+            return $ TA_Unary id ('|':op) x
          <|>
             return x
 
@@ -1816,6 +1816,7 @@ prop_readAssignmentWord8 = isOk readAssignmentWord "a[4''$(cat foo)]=42"
 prop_readAssignmentWord9 = isOk readAssignmentWord "IFS= "
 prop_readAssignmentWord10= isWarning readAssignmentWord "foo$n=42"
 prop_readAssignmentWord11= isOk readAssignmentWord "foo=([a]=b [c] [d]= [e f )"
+prop_readAssignmentWord12= isOk readAssignmentWord "a[b <<= 3 + c]='thing'"
 readAssignmentWord = try $ do
     id <- getNextId
     pos <- getPosition
@@ -1851,14 +1852,10 @@ readAssignmentWord = try $ do
         id <- getNextId
         return $ T_Literal id ""
 
--- This is only approximate. Fixme?
--- * Bash allows foo[' ' "" $(true) 2 ``]=var
--- * foo[bar] dereferences bar
 readArrayIndex = do
     char '['
     optional space
-    x <- readNormalishWord "]"
-    optional space
+    x <- readArithmeticContents
     char ']'
     return x