diff --git a/src/function_group.hh b/src/function_group.hh
index bafdab27..2772389c 100644
--- a/src/function_group.hh
+++ b/src/function_group.hh
@@ -44,15 +44,15 @@ public:
         return *group;
     }
 
-    CandidateList complete_id(const String& prefix, ByteCount cursor_pos)
+    CandidateList complete_id(const String& prefix, ByteCount cursor_pos) const
     {
         return m_functions.complete_id(prefix, cursor_pos);
     }
 
-    CandidateList complete_group_id(const String& prefix, ByteCount cursor_pos)
+    CandidateList complete_group_id(const String& prefix, ByteCount cursor_pos) const
     {
         return m_functions.complete_id_if(
-            prefix, cursor_pos, [](FunctionAndId& func) {
+            prefix, cursor_pos, [](const FunctionAndId& func) {
                 return func.second.template target<FunctionGroup>() != nullptr;
             });
     }
diff --git a/src/idvaluemap.hh b/src/idvaluemap.hh
index e11c327a..b2cebb1d 100644
--- a/src/idvaluemap.hh
+++ b/src/idvaluemap.hh
@@ -66,7 +66,7 @@ public:
     template<typename _Condition>
     CandidateList complete_id_if(const String& prefix,
                                  ByteCount cursor_pos,
-                                 _Condition condition)
+                                 _Condition condition) const
     {
         String real_prefix = prefix.substr(0, cursor_pos);
         CandidateList result;
@@ -83,7 +83,7 @@ public:
     }
 
     CandidateList complete_id(const String& prefix,
-                              ByteCount cursor_pos)
+                              ByteCount cursor_pos) const
     {
         return complete_id_if(
             prefix, cursor_pos, [](const value_type&) { return true; });