diff --git a/lang/compile.py b/lang/compile.py
index 04f0d84..1386ed6 100755
--- a/lang/compile.py
+++ b/lang/compile.py
@@ -973,11 +973,11 @@
 	%sNULL,
 };
 
-static ModuleClassCall CLASSNAME_module_calls[] = {
+static const ModuleClassCall CLASSNAME_module_calls[] = {
 	%s{.name = NULL, /* end */ },
 };
 
-static ModuleClass CLASSNAME_module_class = {
+static const ModuleClass CLASSNAME_module_class = {
 	.fields_count = 0,
 	.bytecodes = CLASSNAME_bytecodes,
 	.calls = CLASSNAME_module_calls,
@@ -1001,7 +1001,7 @@
 %s	NULL, /* end */
 };
 
-static ModuleClass *module_classes[] = {
+static const ModuleClass *module_classes[] = {
 %s	NULL, /* end */
 };
 
diff --git a/lang/module.c b/lang/module.c
index 9d805d8..70d1eba 100644
--- a/lang/module.c
+++ b/lang/module.c
@@ -101,7 +101,7 @@
 	struct module_class_data *data =
 		(struct module_class_data *)object_priv(
 			state, obj, &module_class_class);
-	ModuleClassCall *call = data->class->calls;
+	const ModuleClassCall *call = data->class->calls;
 	while (call->name != NULL) {
 		if (strcmp(call->name, name) == 0) {
 			const unsigned char *bytecode =
diff --git a/lang/module.h b/lang/module.h
index 12896ea..43bdbd3 100644
--- a/lang/module.h
+++ b/lang/module.h
@@ -23,16 +23,16 @@
 // A call the module class can make
 struct module_class_call {
 	const char *name;
-	int bytecode_index;
+	const int bytecode_index;
 };
 
 typedef struct module_class_call ModuleClassCall;
 
 // A class in a module
 struct module_class {
-	int fields_count;
+	const int fields_count;
 	const unsigned char **bytecodes;
-	ModuleClassCall *calls;
+	const ModuleClassCall *calls;
 };
 
 typedef struct module_class ModuleClass;
@@ -41,7 +41,7 @@
 struct module_info {
 	const char *name;
 	const char **uses;
-	ModuleClass **classes;
+	const ModuleClass **classes;
 	Object (*create)(VmState state, Object use_modules);
 };