diff --git a/Math/NumberTheory/Primes/Counting/Impl.hs b/Math/NumberTheory/Primes/Counting/Impl.hs
--- a/Math/NumberTheory/Primes/Counting/Impl.hs
+++ b/Math/NumberTheory/Primes/Counting/Impl.hs
@@ -431,10 +431,10 @@
 countToNth !n (PS v0 bs : more) = go n 0
   where
     wa :: UArray Int Word
-    wa = unsafeCoerce bs
+    wa = coerceArray bs
 
     go !k i
-      | i == snd (bounds wa)
+      | i > snd (bounds wa)
       = countToNth k more
       | otherwise
       = let w = unsafeAt wa i
@@ -450,13 +450,24 @@
 countAll (PS _ bs) = go 0 0
   where
     wa :: UArray Int Word
-    wa = unsafeCoerce bs
+    wa = coerceArray bs
 
     go !ct i
-      | i == snd (bounds wa)
+      | i > snd (bounds wa)
       = ct
       | otherwise
       = go (ct + popCount (unsafeAt wa i)) (i+1)
+
+-- This is dangerous: we rely on the fact that reading a whole word
+-- at the end of ByteArray is permissible, even if its length
+-- is not aligned with word boundaries.
+coerceArray :: UArray Int Bool -> UArray Int Word
+coerceArray (UArray 0 u n ba)
+  | u + 1 == n, n' > 0 = UArray 0 (n' - 1) n' ba
+  where
+    n' = (n + rMASK) `shiftR` wSHFT
+coerceArray (UArray l u n _) =
+    error $ "Cannot coerce array of Bool to array of Word: " ++ show (l, u, n)
 
 -- Find the j-th highest of bc set bits in the Word w.
 top :: Word -> Int -> Int -> Int
diff --git a/arithmoi.cabal b/arithmoi.cabal
--- a/arithmoi.cabal
+++ b/arithmoi.cabal
@@ -1,5 +1,5 @@
 name:          arithmoi
-version:       0.13.0.1
+version:       0.13.1.0
 cabal-version: 2.0
 build-type:    Simple
 license:       MIT
@@ -17,7 +17,7 @@
   powers (integer roots and tests, modular exponentiation).
 category:      Math, Algorithms, Number Theory
 author:        Andrew Lelechenko, Daniel Fischer
-tested-with:   GHC ==9.0.2 GHC ==9.2.8 GHC ==9.4.8 GHC ==9.6.6 GHC ==9.8.4 GHC ==9.10.1 GHC ==9.12.1
+tested-with:   GHC ==9.0.2 GHC ==9.2.8 GHC ==9.4.8 GHC ==9.6.7 GHC ==9.8.4 GHC ==9.10.1 GHC ==9.12.2
 extra-doc-files:
   changelog.md
 
diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## 0.13.1.0
+
+### Fixed
+
+* Fix a grave bug in prime counting, lurking since `arithmoi-0.11.0.0`.
+
 ## 0.13.0.1
 
 ### Fixed
diff --git a/test-suite/Math/NumberTheory/Primes/CountingTests.hs b/test-suite/Math/NumberTheory/Primes/CountingTests.hs
--- a/test-suite/Math/NumberTheory/Primes/CountingTests.hs
+++ b/test-suite/Math/NumberTheory/Primes/CountingTests.hs
@@ -36,6 +36,8 @@
   , (10^10,  455052511)
   , (10^11,  4118054813)
   , (10^12,  37607912018)
+  -- Enable tests below to validate any changes to 'primeCount' implementation:
+  -- certain routines are not triggered until very large numbers.
   -- , (10^13,  346065536839)
   -- , (10^14,  3204941750802)
   -- , (10^15,  29844570422669)