diff --git a/src/allocator.cpp b/src/allocator.cpp
index 83a7229e..39b8a7b9 100644
--- a/src/allocator.cpp
+++ b/src/allocator.cpp
@@ -13,19 +13,19 @@ allocator::allocator(uint64_t blocks)
     {
         throw std::invalid_argument("blocks");
     }
-    uint64_t p2 = 1, total = 1;
+    uint64_t p2 = 1;
+    total = 0;
     while (p2 * 64 < blocks)
     {
-        p2 = p2 * 64;
         total += p2;
+        p2 = p2 * 64;
     }
-    total -= p2;
     total += (blocks+63) / 64;
-    mask = new uint64_t[2 + total];
+    mask = new uint64_t[total];
     size = free = blocks;
     last_one_mask = (blocks % 64) == 0
         ? UINT64_MAX
-        : ~(UINT64_MAX << (64 - blocks % 64));
+        : ((1l << (blocks % 64)) - 1);
     for (uint64_t i = 0; i < total; i++)
     {
         mask[i] = 0;
@@ -99,6 +99,10 @@ uint64_t allocator::find_free()
     uint64_t p2 = 1, offset = 0, addr = 0, f, i;
     while (p2 < size)
     {
+        if (offset+addr >= total)
+        {
+            return UINT64_MAX;
+        }
         uint64_t m = mask[offset + addr];
         for (i = 0, f = 1; i < 64; i++, f <<= 1)
         {
@@ -113,11 +117,6 @@ uint64_t allocator::find_free()
             return UINT64_MAX;
         }
         addr = (addr * 64) | i;
-        if (addr >= size)
-        {
-            // No space
-            return UINT64_MAX;
-        }
         offset += p2;
         p2 = p2 * 64;
     }
diff --git a/src/allocator.h b/src/allocator.h
index 76304340..58b6808d 100644
--- a/src/allocator.h
+++ b/src/allocator.h
@@ -8,6 +8,7 @@
 // Hierarchical bitmap allocator
 class allocator
 {
+    uint64_t total;
     uint64_t size;
     uint64_t free;
     uint64_t last_one_mask;
diff --git a/src/test_allocator.cpp b/src/test_allocator.cpp
index 736d3070..71d9fca5 100644
--- a/src/test_allocator.cpp
+++ b/src/test_allocator.cpp
@@ -2,20 +2,39 @@
 // License: VNPL-1.1 (see README.md for details)
 
 #include <stdio.h>
+#include <stdlib.h>
 #include "allocator.h"
 
+void alloc_all(int size)
+{
+    allocator *a = new allocator(size);
+    for (int i = 0; i < size; i++)
+    {
+        uint64_t x = a->find_free();
+        if (x == UINT64_MAX)
+        {
+            printf("ran out of space %d allocated=%d\n", size, i);
+            exit(1);
+        }
+        if (x != i)
+        {
+            printf("incorrect block allocated: expected %d, got %lu\n", i, x);
+        }
+        a->set(x, true);
+    }
+    uint64_t x = a->find_free();
+    if (x != UINT64_MAX)
+    {
+        printf("extra free space found: %lx (%d)\n", x, size);
+        exit(1);
+    }
+    delete a;
+}
+
 int main(int narg, char *args[])
 {
-    allocator a(8192);
-    for (int i = 0; i < 8192; i++)
-    {
-        uint64_t x = a.find_free();
-        if (x == UINT64_MAX)
-        {
-            printf("ran out of space %d\n", i);
-            return 1;
-        }
-        a.set(x, true);
-    }
+    alloc_all(8192);
+    alloc_all(8062);
+    alloc_all(4096);
     return 0;
 }