diff --git a/src/assert.h b/src/assert.h
new file mode 100644
index 0000000..376b2c7
--- /dev/null
+++ b/src/assert.h
@@ -0,0 +1,10 @@
+#include <kernaux/assert.h>
+
+#ifdef ENABLE_ASSERT
+#define assert(cond) kernaux_assert((cond))
+#else
+#define assert(cond) ((void)sizeof((cond)))
+#endif
+
+#define assert_return(cond)      { assert((cond)); if (!(cond)) return; }
+#define assert_retval(cond, val) { assert((cond)); if (!(cond)) return (val); }
diff --git a/src/pfa.c b/src/pfa.c
index 29be692..70d6a41 100644
--- a/src/pfa.c
+++ b/src/pfa.c
@@ -5,6 +5,8 @@
 #include <kernaux/pfa.h>
 #include <kernaux/stdlib.h>
 
+#include "assert.h"
+
 #define PAGE_INDEX(page_addr) ((page_addr) / KERNAUX_PFA_PAGE_SIZE)
 
 #define FLAG_INDEX_FROM_INDEX(page_index) ((page_index) / 8)
@@ -39,12 +41,14 @@ __attribute__((nonnull));
 
 void KernAux_PFA_initialize(const KernAux_PFA pfa)
 {
+    assert(pfa);
     kernaux_memset(pfa->flags, 0, sizeof(pfa->flags));
 }
 
 bool KernAux_PFA_is_available(const KernAux_PFA pfa, const size_t page_addr)
 {
-    if (page_addr % KERNAUX_PFA_PAGE_SIZE != 0) return false;
+    assert(pfa);
+    assert_retval(page_addr % KERNAUX_PFA_PAGE_SIZE == 0, false);
 
     return GET_FLAG_FROM_ADDR(pfa, page_addr);
 }
@@ -71,7 +75,8 @@ void KernAux_PFA_mark(
     size_t start,
     size_t end
 ) {
-    if (start >= end) return;
+    assert(pfa);
+    assert_return(start < end);
 
     const size_t start_rem = start % KERNAUX_PFA_PAGE_SIZE;
     const size_t end_rem = (end + 1) % KERNAUX_PFA_PAGE_SIZE;
@@ -98,6 +103,8 @@ void KernAux_PFA_mark(
 
 size_t KernAux_PFA_alloc_pages(const KernAux_PFA pfa, size_t mem_size)
 {
+    assert(pfa);
+
     const size_t mem_rem = mem_size % KERNAUX_PFA_PAGE_SIZE;
 
     if (mem_rem != 0) {
@@ -135,7 +142,8 @@ void KernAux_PFA_free_pages(
     const size_t page_addr,
     size_t mem_size
 ) {
-    if (page_addr % KERNAUX_PFA_PAGE_SIZE != 0) return;
+    assert(pfa);
+    assert_return(page_addr % KERNAUX_PFA_PAGE_SIZE == 0);
 
     const size_t mem_rem = mem_size % KERNAUX_PFA_PAGE_SIZE;