diff --git a/src/datastructures.c b/src/datastructures.c
index 16b30fcffcd3d5fad40f7271e666df6b9b8994bb..5cb0fb4224f2bfd40abc6f686c7e528a3f17ad5e 100644
--- a/src/datastructures.c
+++ b/src/datastructures.c
@@ -116,7 +116,7 @@ HHashTable* h_hashtable_new(HArena *arena, HEqualFunc equalFunc, HHashFunc hashF
   return ht;
 }
 
-void* h_hashtable_get(HHashTable* ht, const void* key) {
+void* h_hashtable_get(const HHashTable* ht, const void* key) {
   HHashValue hashval = ht->hashFunc(key);
 #ifdef CONSISTENCY_CHECK
   assert((ht->capacity & (ht->capacity - 1)) == 0); // capacity is a power of 2
@@ -180,7 +180,7 @@ void h_hashtable_update(HHashTable *dst, const HHashTable *src) {
   }
 }
 
-int   h_hashtable_present(HHashTable* ht, const void* key) {
+int   h_hashtable_present(const HHashTable* ht, const void* key) {
   HHashValue hashval = ht->hashFunc(key);
 #ifdef CONSISTENCY_CHECK
   assert((ht->capacity & (ht->capacity - 1)) == 0); // capacity is a power of 2
diff --git a/src/internal.h b/src/internal.h
index 6870b780d00d9eaf22e232efcc644c90fc488ea0..4c30eb535b84130c912490a44333d411e0117f6b 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -248,13 +248,13 @@ HSlist* h_slist_remove_all(HSlist *slist, const void* item);
 void h_slist_free(HSlist *slist);
 
 HHashTable* h_hashtable_new(HArena *arena, HEqualFunc equalFunc, HHashFunc hashFunc);
-void* h_hashtable_get(HHashTable* ht, const void* key);
+void* h_hashtable_get(const HHashTable* ht, const void* key);
 void  h_hashtable_put(HHashTable* ht, const void* key, void* value);
 void  h_hashtable_update(HHashTable* dst, const HHashTable *src);
-int   h_hashtable_present(HHashTable* ht, const void* key);
+int   h_hashtable_present(const HHashTable* ht, const void* key);
 void  h_hashtable_del(HHashTable* ht, const void* key);
 void  h_hashtable_free(HHashTable* ht);
-static inline bool h_hashtable_empty(HHashTable* ht) { return (ht->used == 0); }
+static inline bool h_hashtable_empty(const HHashTable* ht) { return (ht->used == 0); }
 
 typedef HHashTable HHashSet;
 #define h_hashset_new(a,eq,hash) h_hashtable_new(a,eq,hash)