diff --git a/src/builtin_complete.cpp b/src/builtin_complete.cpp
index 8f33632bb..8ae0bb528 100644
--- a/src/builtin_complete.cpp
+++ b/src/builtin_complete.cpp
@@ -303,11 +303,10 @@ int builtin_complete(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
         prefix.append(cmd);
         prefix.append(L": ");
 
-        wcstring err_text;
-        if (parser.detect_errors_in_argument_list(comp, &err_text, prefix.c_str())) {
+        if (maybe_t<wcstring> err_text = parse_util_detect_errors_in_argument_list(comp, prefix)) {
             streams.err.append_format(L"%ls: Completion '%ls' contained a syntax error\n", cmd,
                                       comp);
-            streams.err.append(err_text);
+            streams.err.append(*err_text);
             streams.err.push_back(L'\n');
             return STATUS_CMD_ERROR;
         }
diff --git a/src/parse_util.cpp b/src/parse_util.cpp
index 7f5c14f05..b4348f7d3 100644
--- a/src/parse_util.cpp
+++ b/src/parse_util.cpp
@@ -1333,3 +1333,35 @@ parser_test_error_bits_t parse_util_detect_errors(const wcstring &buff_src,
 
     return res;
 }
+
+maybe_t<wcstring> parse_util_detect_errors_in_argument_list(const wcstring &arg_list_src,
+                                                            const wcstring &prefix) {
+    // Helper to return a description of the first error.
+    auto get_error_text = [&](const parse_error_list_t &errors) {
+        assert(!errors.empty() && "Expected an error");
+        return errors.at(0).describe_with_prefix(arg_list_src, prefix, false /* not interactive */,
+                                                 false /* don't skip caret */);
+    };
+
+    // Parse the string as an argument list.
+    parse_error_list_t errors;
+    parse_node_tree_t tree;
+    if (!parse_tree_from_string(arg_list_src, parse_flag_none, &tree, &errors,
+                                symbol_freestanding_argument_list)) {
+        // Failed to parse.
+        return get_error_text(errors);
+    }
+
+    // Get the root argument list and extract arguments from it.
+    // Test each of these.
+    assert(!tree.empty() && "Should have parsed a tree");
+    tnode_t<grammar::freestanding_argument_list> arg_list(&tree, &tree.at(0));
+    while (auto arg = arg_list.next_in_list<grammar::argument>()) {
+        const wcstring arg_src = arg.get_source(arg_list_src);
+        if (parse_util_detect_errors_in_argument(arg, arg_src, &errors)) {
+            return get_error_text(errors);
+        }
+    }
+
+    return none();
+}
diff --git a/src/parse_util.h b/src/parse_util.h
index 74438544c..e862b796f 100644
--- a/src/parse_util.h
+++ b/src/parse_util.h
@@ -132,6 +132,11 @@ parser_test_error_bits_t parse_util_detect_errors(const wcstring &buff_src,
                                                   bool allow_incomplete = true,
                                                   parsed_source_ref_t *out_pstree = NULL);
 
+/// Detect errors in the specified string when parsed as an argument list. Returns the text of an
+/// error, or none if no error occurred.
+maybe_t<wcstring> parse_util_detect_errors_in_argument_list(const wcstring &arg_list_src,
+                                                            const wcstring &prefix = {});
+
 /// Test if this argument contains any errors. Detected errors include syntax errors in command
 /// substitutions, improperly escaped characters and improper use of the variable expansion
 /// operator. This does NOT currently detect unterminated quotes.
diff --git a/src/parser.cpp b/src/parser.cpp
index 73d7e10b7..bfaf9a003 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -680,41 +680,6 @@ template int parser_t::eval_node(parsed_source_ref_t, tnode_t<grammar::job_list>
                                  const io_chain_t &, enum block_type_t,
                                  std::shared_ptr<job_t> parent_job);
 
-bool parser_t::detect_errors_in_argument_list(const wcstring &arg_list_src, wcstring *out,
-                                              const wchar_t *prefix) const {
-    bool errored = false;
-    parse_error_list_t errors;
-
-    // Use empty string for the prefix if it's NULL.
-    if (!prefix) prefix = L"";  //!OCLINT(parameter reassignment)
-
-    // Parse the string as an argument list.
-    parse_node_tree_t tree;
-    if (!parse_tree_from_string(arg_list_src, parse_flag_none, &tree, &errors,
-                                symbol_freestanding_argument_list)) {
-        // Failed to parse.
-        errored = true;
-    }
-
-    if (!errored) {
-        // Get the root argument list and extract arguments from it.
-        assert(!tree.empty());  //!OCLINT(multiple unary operator)
-        tnode_t<grammar::freestanding_argument_list> arg_list(&tree, &tree.at(0));
-        while (auto arg = arg_list.next_in_list<grammar::argument>()) {
-            const wcstring arg_src = arg.get_source(arg_list_src);
-            if (parse_util_detect_errors_in_argument(arg, arg_src, &errors)) {
-                errored = true;
-            }
-        }
-    }
-
-    if (!errors.empty() && out != NULL) {
-        out->assign(errors.at(0).describe_with_prefix(
-            arg_list_src, prefix, false /* not interactive */, false /* don't skip caret */));
-    }
-    return errored;
-}
-
 void parser_t::get_backtrace(const wcstring &src, const parse_error_list_t &errors,
                              wcstring &output) const {
     if (!errors.empty()) {
diff --git a/src/parser.h b/src/parser.h
index c522be72d..09842702d 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -335,11 +335,6 @@ class parser_t : public std::enable_shared_from_this<parser_t> {
     void get_backtrace(const wcstring &src, const parse_error_list_t &errors,
                        wcstring &output) const;
 
-    /// Detect errors in the specified string when parsed as an argument list. Returns true if an
-    /// error occurred.
-    bool detect_errors_in_argument_list(const wcstring &arg_list_src, wcstring *out_err,
-                                        const wchar_t *prefix) const;
-
     /// Tell the parser that the specified function may not be run if not inside of a conditional
     /// block. This is to remove some possibilities of infinite recursion.
     void forbid_function(const wcstring &function);