diff --git a/src/options.c b/src/options.c
index 33ea0a1f..cfb15aec 100644
--- a/src/options.c
+++ b/src/options.c
@@ -240,7 +240,7 @@ void mi_register_output(mi_output_fun* out, void* arg) mi_attr_noexcept {
 }
 
 // add stderr to the delayed output after the module is loaded
-static void mi_add_stderr_output() {
+static void mi_add_stderr_output(void) {
   mi_assert_internal(mi_out_default == NULL);
   mi_out_buf_flush(&mi_out_stderr, false, NULL); // flush current contents to stderr
   mi_out_default = &mi_out_buf_stderr;           // and add stderr to the delayed output
diff --git a/test/test-api.c b/test/test-api.c
index c78e1972..852e3e8a 100644
--- a/test/test-api.c
+++ b/test/test-api.c
@@ -286,7 +286,7 @@ int main(void) {
 // Larger test functions
 // ---------------------------------------------------
 
-bool test_heap1() {
+bool test_heap1(void) {
   mi_heap_t* heap = mi_heap_new();
   int* p1 = mi_heap_malloc_tp(heap,int);
   int* p2 = mi_heap_malloc_tp(heap,int);
@@ -295,7 +295,7 @@ bool test_heap1() {
   return true;
 }
 
-bool test_heap2() {
+bool test_heap2(void) {
   mi_heap_t* heap = mi_heap_new();
   int* p1 = mi_heap_malloc_tp(heap,int);
   int* p2 = mi_heap_malloc_tp(heap,int);
@@ -306,7 +306,7 @@ bool test_heap2() {
   return true;
 }
 
-bool test_stl_allocator1() {
+bool test_stl_allocator1(void) {
 #ifdef __cplusplus
   std::vector<int, mi_stl_allocator<int> > vec;
   vec.push_back(1);
@@ -319,7 +319,7 @@ bool test_stl_allocator1() {
 
 struct some_struct  { int i; int j; double z; };
 
-bool test_stl_allocator2() {
+bool test_stl_allocator2(void) {
 #ifdef __cplusplus
   std::vector<some_struct, mi_stl_allocator<some_struct> > vec;
   vec.push_back(some_struct());