diff --git a/src/greaseweazle/track.py b/src/greaseweazle/track.py
index cc05f54b..537bbaf5 100644
--- a/src/greaseweazle/track.py
+++ b/src/greaseweazle/track.py
@@ -170,7 +170,7 @@ def _flux(self, for_writeout, cue_at_index) -> Union[Flux, WriteoutFlux]:
         # Weak regions need special processing for correct flux representation.
         for s,n in self.weak:
             e = s + n
-            assert 0 < s < e < bitlen
+            assert 0 < s < e <= bitlen
             pattern = bitarray(endian="big")
             if n < 400 or self.force_random_weak:
                 # Short weak regions are written with no flux transitions.
@@ -194,7 +194,7 @@ def _flux(self, for_writeout, cue_at_index) -> Union[Flux, WriteoutFlux]:
             # clocked out a 0.
             bits[s] = not bits[s-1]
             # Similarly modify the last bit of the weak region.
-            bits[e-1] = not(bits[e-2] or bits[e])
+            bits[e-1] = not(bits[e-2] or bits[e % bitlen])
 
         if cue_at_index:
             # Rotate data to start at the index.