diff --git a/src/plugins_types.c b/src/plugins_types.c
index 4441f4f..4420938 100644
--- a/src/plugins_types.c
+++ b/src/plugins_types.c
@@ -44,7 +44,7 @@
  * @brief Find import prefix in imports.
  */
 static const struct lys_module *
-ly_schema_resolve_prefix(const struct ly_ctx *UNUSED(ctx), const char *prefix, size_t prefix_len, void *prefix_data)
+ly_schema_resolve_prefix(const struct ly_ctx *UNUSED(ctx), const char *prefix, size_t prefix_len, const void *prefix_data)
 {
     const struct lysp_module *prefix_mod = prefix_data;
     struct lys_module *m = NULL;
@@ -75,9 +75,9 @@
  */
 static const struct lys_module *
 ly_schema_resolved_resolve_prefix(const struct ly_ctx *UNUSED(ctx), const char *prefix, size_t prefix_len,
-        void *prefix_data)
+        const void *prefix_data)
 {
-    struct lysc_prefix *prefixes = prefix_data;
+    const struct lysc_prefix *prefixes = prefix_data;
     LY_ARRAY_COUNT_TYPE u;
 
     LY_ARRAY_FOR(prefixes, u) {
@@ -93,7 +93,7 @@
  * @brief Find XML namespace prefix in XML namespaces, which are then mapped to modules.
  */
 static const struct lys_module *
-ly_xml_resolve_prefix(const struct ly_ctx *ctx, const char *prefix, size_t prefix_len, void *prefix_data)
+ly_xml_resolve_prefix(const struct ly_ctx *ctx, const char *prefix, size_t prefix_len, const void *prefix_data)
 {
     const struct lys_module *mod;
     const struct lyxml_ns *ns;
@@ -116,13 +116,14 @@
  * @brief Find module name.
  */
 static const struct lys_module *
-ly_json_resolve_prefix(const struct ly_ctx *ctx, const char *prefix, size_t prefix_len, void *UNUSED(prefix_data))
+ly_json_resolve_prefix(const struct ly_ctx *ctx, const char *prefix, size_t prefix_len, const void *UNUSED(prefix_data))
 {
     return ly_ctx_get_module_implemented2(ctx, prefix, prefix_len);
 }
 
 const struct lys_module *
-ly_resolve_prefix(const struct ly_ctx *ctx, const char *prefix, size_t prefix_len, LY_PREFIX_FORMAT format, void *prefix_data)
+ly_resolve_prefix(const struct ly_ctx *ctx, const char *prefix, size_t prefix_len, LY_PREFIX_FORMAT format,
+        const void *prefix_data)
 {
     const struct lys_module *mod = NULL;
 
@@ -148,7 +149,7 @@
 
 API const struct lys_module *
 ly_type_identity_module(const struct ly_ctx *ctx, const struct lysc_node *ctx_node,
-        const char *prefix, size_t prefix_len, LY_PREFIX_FORMAT format, void *prefix_data)
+        const char *prefix, size_t prefix_len, LY_PREFIX_FORMAT format, const void *prefix_data)
 {
     if (prefix_len) {
         return ly_resolve_prefix(ctx, prefix, prefix_len, format, prefix_data);
diff --git a/src/plugins_types.h b/src/plugins_types.h
index fdff766..2d139c5 100644
--- a/src/plugins_types.h
+++ b/src/plugins_types.h
@@ -143,7 +143,7 @@
  * @return NULL otherwise.
  */
 const struct lys_module *ly_type_identity_module(const struct ly_ctx *ctx, const struct lysc_node *ctx_node,
-        const char *prefix, size_t prefix_len, LY_PREFIX_FORMAT format, void *prefix_data);
+        const char *prefix, size_t prefix_len, LY_PREFIX_FORMAT format, const void *prefix_data);
 
 /**
  * @brief Implement a module (just like ::lys_set_implemented()), but keep maintaining unresolved items.
diff --git a/src/tree_data_helpers.c b/src/tree_data_helpers.c
index 4632d50..76b890a 100644
--- a/src/tree_data_helpers.c
+++ b/src/tree_data_helpers.c
@@ -544,7 +544,7 @@
 
 LY_ERR
 ly_store_prefix_data(const struct ly_ctx *ctx, const char *value, size_t value_len, LY_PREFIX_FORMAT format,
-        void *prefix_data, LY_PREFIX_FORMAT *format_p, void **prefix_data_p)
+        const void *prefix_data, LY_PREFIX_FORMAT *format_p, void **prefix_data_p)
 {
     LY_ERR ret = LY_SUCCESS;
     const char *start, *stop;
diff --git a/src/tree_data_internal.h b/src/tree_data_internal.h
index 184baf6..402f732 100644
--- a/src/tree_data_internal.h
+++ b/src/tree_data_internal.h
@@ -460,7 +460,7 @@
  * @return LY_ERR value.
  */
 LY_ERR ly_store_prefix_data(const struct ly_ctx *ctx, const char *value, size_t value_len, LY_PREFIX_FORMAT format,
-        void *prefix_data, LY_PREFIX_FORMAT *format_p, void **prefix_data_p);
+        const void *prefix_data, LY_PREFIX_FORMAT *format_p, void **prefix_data_p);
 
 /**
  * @brief Get string name of the format.
diff --git a/src/tree_schema_internal.h b/src/tree_schema_internal.h
index e226a99..8804f1b 100644
--- a/src/tree_schema_internal.h
+++ b/src/tree_schema_internal.h
@@ -836,6 +836,6 @@
  * @return NULL otherwise.
  */
 const struct lys_module *ly_resolve_prefix(const struct ly_ctx *ctx, const char *prefix, size_t prefix_len,
-        LY_PREFIX_FORMAT format, void *prefix_data);
+        LY_PREFIX_FORMAT format, const void *prefix_data);
 
 #endif /* LY_TREE_SCHEMA_INTERNAL_H_ */