diff --git a/thumbs.c b/thumbs.c
index c9e97c4..038b501 100644
--- a/thumbs.c
+++ b/thumbs.c
@@ -21,6 +21,7 @@
 #define INCLUDE_THUMBS_CONFIG
 #include "config.h"
 
+#include <assert.h>
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -40,8 +41,7 @@ static char* tns_cache_filepath(const char *filepath)
 	size_t len;
 	char *cfile = NULL;
 
-	if (*filepath != '/')
-		return NULL;
+	assert(*filepath == '/' && "filepath should be result of realpath(3)");
 
 	if (strncmp(filepath, cache_dir, strlen(cache_dir)) != 0) {
 		/* don't cache images inside the cache directory! */
@@ -355,9 +355,7 @@ void tns_unload(tns_t *tns, int n)
 {
 	thumb_t *t;
 
-	if (n < 0 || n >= *tns->cnt)
-		return;
-
+	assert(n >= 0 && n < *tns->cnt);
 	t = &tns->thumbs[n];
 
 	if (t->im != NULL) {
@@ -371,9 +369,7 @@ static void tns_check_view(tns_t *tns, bool scrolled)
 {
 	int r;
 
-	if (tns == NULL)
-		return;
-
+	assert(tns != NULL);
 	tns->first -= tns->first % tns->cols;
 	r = *tns->sel % tns->cols;
 
diff --git a/util.c b/util.c
index cfe64c6..94d4247 100644
--- a/util.c
+++ b/util.c
@@ -226,8 +226,7 @@ void construct_argv(char **argv, unsigned int len, ...)
 	for (i = 0; i < len; ++i)
 		argv[i] = va_arg(args, char *);
 	va_end(args);
-	if (argv[len-1] != NULL)
-		error(EXIT_FAILURE, 0, "argv not NULL terminated");
+	assert(argv[len-1] == NULL && "argv should be NULL terminated");
 }
 
 static int mkspawn_pipe(posix_spawn_file_actions_t *fa, const char *cmd, int *pfd, int dupidx)
diff --git a/window.c b/window.c
index 136b452..6e62979 100644
--- a/window.c
+++ b/window.c
@@ -22,6 +22,7 @@
 #include "config.h"
 #include "icon/data.h"
 
+#include <assert.h>
 #include <locale.h>
 #include <stdlib.h>
 #include <string.h>
@@ -450,10 +451,10 @@ static void win_draw_bar(win_t *win)
 	win_bar_t *l, *r;
 	XftDraw *d;
 
-	if ((l = &win->bar.l)->buf == NULL || (r = &win->bar.r)->buf == NULL)
-		return;
-
 	e = &win->env;
+	l = &win->bar.l;
+	r = &win->bar.r;
+	assert(l->buf != NULL && r->buf != NULL);
 	y = (win->bar.top ? 0 : win->h) + font->ascent + V_TEXT_PAD;
 	w = win->w - 2*H_TEXT_PAD;
 	d = XftDrawCreate(e->dpy, win->buf.pm, e->vis, e->cmap);