diff --git a/headers/private/kernel/vfs.h b/headers/private/kernel/vfs.h
index cdde48d47c..89d98b5801 100644
--- a/headers/private/kernel/vfs.h
+++ b/headers/private/kernel/vfs.h
@@ -176,7 +176,7 @@ int			_user_open_dir_entry_ref(dev_t device, ino_t inode,
 				const char *name);
 int			_user_open_dir(int fd, const char *path);
 int			_user_open_parent_dir(int fd, char *name, size_t nameLength);
-status_t	_user_fcntl(int fd, int op, uint32 argument);
+status_t	_user_fcntl(int fd, int op, size_t argument);
 status_t	_user_fsync(int fd);
 status_t	_user_flock(int fd, int op);
 status_t	_user_read_stat(int fd, const char *path, bool traverseLink,
diff --git a/headers/private/system/syscalls.h b/headers/private/system/syscalls.h
index 17f9b73d79..b104bb6cf8 100644
--- a/headers/private/system/syscalls.h
+++ b/headers/private/system/syscalls.h
@@ -253,7 +253,7 @@ extern int			_kern_open_dir_entry_ref(dev_t device, ino_t inode,
 extern int			_kern_open_dir(int fd, const char *path);
 extern int			_kern_open_parent_dir(int fd, char *name,
 						size_t nameLength);
-extern status_t		_kern_fcntl(int fd, int op, uint32 argument);
+extern status_t		_kern_fcntl(int fd, int op, size_t argument);
 extern status_t		_kern_fsync(int fd);
 extern status_t		_kern_flock(int fd, int op);
 extern off_t		_kern_seek(int fd, off_t pos, int seekType);
diff --git a/src/system/kernel/fs/vfs.cpp b/src/system/kernel/fs/vfs.cpp
index 6e4dfd2d39..ace23089f2 100644
--- a/src/system/kernel/fs/vfs.cpp
+++ b/src/system/kernel/fs/vfs.cpp
@@ -5917,7 +5917,7 @@ common_ioctl(struct file_descriptor* descriptor, ulong op, void* buffer,
 
 
 static status_t
-common_fcntl(int fd, int op, uint32 argument, bool kernel)
+common_fcntl(int fd, int op, size_t argument, bool kernel)
 {
 	struct flock flock;
 
@@ -5934,10 +5934,9 @@ common_fcntl(int fd, int op, uint32 argument, bool kernel)
 	status_t status = B_OK;
 
 	if (op == F_SETLK || op == F_SETLKW || op == F_GETLK) {
-		// TODO: x86_64 needs ulong argument here.
 		if (descriptor->type != FDTYPE_FILE)
 			status = B_BAD_VALUE;
-		else if (user_memcpy(&flock, (struct flock*)((ulong)argument),
+		else if (user_memcpy(&flock, (struct flock*)argument,
 				sizeof(struct flock)) != B_OK)
 			status = B_BAD_ADDRESS;
 
@@ -6019,8 +6018,7 @@ common_fcntl(int fd, int op, uint32 argument, bool kernel)
 				status = get_advisory_lock(vnode, &flock);
 				if (status == B_OK) {
 					// copy back flock structure
-					// TODO: x86_64
-					status = user_memcpy((struct flock*)((ulong)argument), &flock,
+					status = user_memcpy((struct flock*)argument, &flock,
 						sizeof(struct flock));
 				}
 			} else
@@ -8087,7 +8085,7 @@ _kern_open_dir(int fd, const char* path)
 
 
 status_t
-_kern_fcntl(int fd, int op, uint32 argument)
+_kern_fcntl(int fd, int op, size_t argument)
 {
 	return common_fcntl(fd, op, argument, true);
 }
@@ -8901,7 +8899,7 @@ _user_open_parent_dir(int fd, char* userName, size_t nameLength)
 
 
 status_t
-_user_fcntl(int fd, int op, uint32 argument)
+_user_fcntl(int fd, int op, size_t argument)
 {
 	status_t status = common_fcntl(fd, op, argument, false);
 	if (op == F_SETLKW)
diff --git a/src/system/libroot/posix/fcntl.cpp b/src/system/libroot/posix/fcntl.cpp
index 2f99596145..1a82dec027 100644
--- a/src/system/libroot/posix/fcntl.cpp
+++ b/src/system/libroot/posix/fcntl.cpp
@@ -65,7 +65,7 @@ fcntl(int fd, int op, ...)
 {
 	va_list args;
 	va_start(args, op);
-	uint32 argument = va_arg(args, uint32);
+	size_t argument = va_arg(args, size_t);
 	va_end(args);
 
 	status_t error = _kern_fcntl(fd, op, argument);