diff --git a/dev/util_gen_stub_file.py b/dev/util_gen_stub_file.py
index 23287696..985ad95d 100644
--- a/dev/util_gen_stub_file.py
+++ b/dev/util_gen_stub_file.py
@@ -37,6 +37,7 @@
 }
 skip = {
     "__firstlineno__",
+    "__replace__",
     "__builtins__",
     "__cached__",
     "__getstate__",
diff --git a/src/stim/simulators/frame_simulator.pybind.cc b/src/stim/simulators/frame_simulator.pybind.cc
index c4c2c6d8..7c10db55 100644
--- a/src/stim/simulators/frame_simulator.pybind.cc
+++ b/src/stim/simulators/frame_simulator.pybind.cc
@@ -39,10 +39,10 @@ static void generate_biased_samples_bit_packed_contiguous(uint8_t *out, size_t n
     uintptr_t aligned64_start = start & ~0b111ULL;
     uintptr_t aligned64_end = end & ~0b111ULL;
     if (aligned64_start != start) {
-        aligned64_start += 1;
+        aligned64_start += 8;
     }
     biased_randomize_bits(p, (uint64_t *)aligned64_start, (uint64_t *)aligned64_end, rng);
-    if (start != aligned64_start) {
+    if (start < aligned64_start) {
         uint64_t pad;
         biased_randomize_bits(p, &pad, &pad + 1, rng);
         for (size_t k = 0; k < aligned64_start - start; k++) {
@@ -50,11 +50,12 @@ static void generate_biased_samples_bit_packed_contiguous(uint8_t *out, size_t n
             pad >>= 8;
         }
     }
-    if (end != aligned64_end) {
+    if (aligned64_end < end) {
         uint64_t pad;
         biased_randomize_bits(p, &pad, &pad + 1, rng);
-        for (size_t k = 0; k < end - aligned64_end; k++) {
-            ((uint8_t *)aligned64_end)[k] = (uint8_t)(pad & 0xFF);
+        while (aligned64_end < end) {
+            *(uint8_t *)aligned64_end = (uint8_t)(pad & 0xFF);
+            aligned64_end++;
             pad >>= 8;
         }
     }
@@ -119,7 +120,7 @@ pybind11::object generate_bernoulli_samples(FrameSimulator<W> &self, size_t num_
         auto stride = buf.strides(0);
         void *start_of_data = (void *)buf.mutable_data();
 
-        if (stride == 1 && num_bytes > 0) {
+        if (stride == 1) {
             generate_biased_samples_bit_packed_contiguous(
                 (uint8_t *)start_of_data,
                 num_bytes,
diff --git a/src/stim/simulators/frame_simulator_pybind_test.py b/src/stim/simulators/frame_simulator_pybind_test.py
index 92bb30ca..bda0bbfa 100644
--- a/src/stim/simulators/frame_simulator_pybind_test.py
+++ b/src/stim/simulators/frame_simulator_pybind_test.py
@@ -583,6 +583,17 @@ def test_generate_bernoulli_samples():
     assert np.sum(np.unpackbits(v, count=1001, bitorder='little')) == 1001
     assert np.sum(np.unpackbits(v, count=1008, bitorder='little')) == 1001
 
+    v = sim.generate_bernoulli_samples(256, p=0, bit_packed=True)
+    assert np.all(v == 0)
+
+    sim.generate_bernoulli_samples(256 - 101, p=1, bit_packed=True, out=v[1:-11])
+    for k in v:
+        print(k)
+    assert np.all(v[1:-12] == 0xFF)
+    assert v[-12] == 7
+    assert np.all(v[-11:] == 0)
+    assert np.all(v[:1] == 0)
+
     v = sim.generate_bernoulli_samples(2**16, p=0.25, bit_packed=True)
     assert abs(np.sum(np.unpackbits(v, count=2**16)) - 2**16*0.25) < 2**12
 
@@ -591,7 +602,6 @@ def test_generate_bernoulli_samples():
     assert np.all(v[:-1] == 0xFF)
     assert v[-1] == 0x7F
 
-
     v[:] = 0
     sim.generate_bernoulli_samples(2**15, p=1, bit_packed=True, out=v[::2])
     assert np.all(v[0::2] == 0xFF)