diff --git a/lldb/packages/Python/lldbsuite/test/functionalities/completion/TestCompletion.py b/lldb/packages/Python/lldbsuite/test/functionalities/completion/TestCompletion.py
index 2df341e572d04..969a7b543e09f 100644
--- a/lldb/packages/Python/lldbsuite/test/functionalities/completion/TestCompletion.py
+++ b/lldb/packages/Python/lldbsuite/test/functionalities/completion/TestCompletion.py
@@ -90,6 +90,13 @@ def test_process_launch_arch(self):
                               ['mips',
                                'arm64'])
 
+    @skipIfFreeBSD  # timing out on the FreeBSD buildbot
+    def test_ambiguous_long_opt(self):
+        self.completions_match('breakpoint modify --th',
+                               ['--thread-id',
+                                '--thread-index',
+                                '--thread-name'])
+
     @skipIfFreeBSD  # timing out on the FreeBSD buildbot
     def test_plugin_load(self):
         self.complete_from_to('plugin load ', [])
diff --git a/lldb/packages/Python/lldbsuite/test/lldbtest.py b/lldb/packages/Python/lldbsuite/test/lldbtest.py
index 9d38909d679ff..6c00350167d0e 100644
--- a/lldb/packages/Python/lldbsuite/test/lldbtest.py
+++ b/lldb/packages/Python/lldbsuite/test/lldbtest.py
@@ -2194,6 +2194,16 @@ def complete_from_to(self, str_input, patterns, turn_off_re_match=False):
                     compare_string, msg=COMPLETION_MSG(
                         str_input, p, match_strings), exe=False, patterns=[p])
 
+    def completions_match(self, command, completions):
+        """Checks that the completions for the given command are equal to the
+        given list of completions"""
+        interp = self.dbg.GetCommandInterpreter()
+        match_strings = lldb.SBStringList()
+        interp.HandleCompletion(command, len(command), 0, -1, match_strings)
+        # match_strings is a 1-indexed list, so we have to slice...
+        self.assertItemsEqual(completions, list(match_strings)[1:],
+                              "List of returned completion is wrong")
+
     def filecheck(
             self,
             command,
diff --git a/lldb/source/Interpreter/Options.cpp b/lldb/source/Interpreter/Options.cpp
index 2829a22b7e497..1a713ed7fbb54 100644
--- a/lldb/source/Interpreter/Options.cpp
+++ b/lldb/source/Interpreter/Options.cpp
@@ -706,17 +706,11 @@ bool Options::HandleOptionCompletion(CompletionRequest &request,
         // elements
         // that are not unique up to this point.  getopt_long_only does
         // shortest unique match for long options already.
-
-        if (cur_opt_str.startswith("--")) {
+        if (cur_opt_str.consume_front("--")) {
           for (auto &def : opt_defs) {
-            if (!def.long_option)
-              continue;
-
-            if (cur_opt_str.startswith(def.long_option)) {
-              std::string full_name("--");
-              full_name.append(def.long_option);
-              request.AddCompletion(full_name);
-            }
+            llvm::StringRef long_option(def.long_option);
+            if (long_option.startswith(cur_opt_str))
+              request.AddCompletion("--" + long_option.str());
           }
         }
         return true;