diff --git a/src/expand.cpp b/src/expand.cpp
index 396745476..4473b481f 100644
--- a/src/expand.cpp
+++ b/src/expand.cpp
@@ -649,7 +649,12 @@ static expand_result_t expand_cmdsubst(wcstring input, parser_t &parser,
 
         bad_pos = parse_slice(slice_begin, &slice_end, slice_idx, sub_res.size());
         if (bad_pos != 0) {
-            append_syntax_error(errors, slice_begin - in + bad_pos, L"Invalid index value");
+            if (tail_begin[bad_pos] == L'0') {
+                append_syntax_error(errors, slice_begin - in + bad_pos,
+                                    L"array indices start at 1, not 0.");
+            } else {
+                append_syntax_error(errors, slice_begin - in + bad_pos, L"Invalid index value");
+            }
             return expand_result_t::make_error(STATUS_EXPAND_ERROR);
         }