diff --git a/src/core/abstract.c b/src/core/abstract.c
index 1e218efb..31ddb50c 100644
--- a/src/core/abstract.c
+++ b/src/core/abstract.c
@@ -26,12 +26,13 @@
 #include "util.h"
 #include "gc.h"
 #include "state.h"
+#endif
+
 #ifdef JANET_EV
 #ifdef JANET_WINDOWS
 #include <windows.h>
 #endif
 #endif
-#endif
 
 /* Create new userdata */
 void *janet_abstract_begin(const JanetAbstractType *atype, size_t size) {
@@ -86,11 +87,11 @@ void *janet_abstract_threaded(const JanetAbstractType *atype, size_t size) {
 
 #ifdef JANET_WINDOWS
 
-void janet_os_mutex_size(void) {
+size_t janet_os_mutex_size(void) {
     return sizeof(CRITICAL_SECTION);
 }
 
-void janet_os_rwlock_size(void) {
+size_t janet_os_rwlock_size(void) {
     return sizeof(SRWLock);
 }