diff --git a/src/ShellCheck/Analytics.hs b/src/ShellCheck/Analytics.hs
index 2888047..cf8015b 100644
--- a/src/ShellCheck/Analytics.hs
+++ b/src/ShellCheck/Analytics.hs
@@ -1298,7 +1298,7 @@ checkArithmeticDeref params t@(TA_Expansion _ [b@(T_DollarBraced id _ _)]) =
     unless (isException $ bracedString b) getWarning
   where
     isException [] = True
-    isException s = any (`elem` "/.:#%?*@$-!+=^,") s || isDigit (head s)
+    isException s@(h:_) = any (`elem` "/.:#%?*@$-!+=^,") s || isDigit h
     getWarning = fromMaybe noWarning . msum . map warningFor $ parents params t
     warningFor t =
         case t of