diff --git a/clash-vexriscv/clash-vexriscv.cabal b/clash-vexriscv/clash-vexriscv.cabal
index 7ae96cb..fe48780 100644
--- a/clash-vexriscv/clash-vexriscv.cabal
+++ b/clash-vexriscv/clash-vexriscv.cabal
@@ -108,6 +108,7 @@ library
   hs-source-dirs: src
   default-language: Haskell2010
   exposed-modules:
+    VexRiscv.Random
     VexRiscv
     VexRiscv.ClockTicks
     VexRiscv.FFI
@@ -126,6 +127,7 @@ library
     Glob,
     network,
     process >= 1.6 && < 1.8,
+    random,
     string-interpolate,
     tagged,
     template-haskell,
@@ -141,6 +143,7 @@ test-suite unittests
   other-modules:
     Tests.Extra
     Tests.VexRiscv.ClockTicks
+    Tests.VexRiscv.Random
   build-depends:
     HUnit,
     base,
diff --git a/clash-vexriscv/src/VexRiscv.hs b/clash-vexriscv/src/VexRiscv.hs
index 098aba3..3b0c03d 100644
--- a/clash-vexriscv/src/VexRiscv.hs
+++ b/clash-vexriscv/src/VexRiscv.hs
@@ -33,6 +33,7 @@ import VexRiscv.ClockTicks
 import VexRiscv.FFI
 import VexRiscv.TH
 import VexRiscv.VecToTuple
+import VexRiscv.Random
 
 import qualified VexRiscv.FFI as FFI
 
@@ -259,23 +260,39 @@ vexRiscv# !_sourcePath clk rst0
   jtag_TDI = unsafePerformIO $ do
     (v, initStage1, initStage2, stepRising, stepFalling, _shutDown) <- vexCPU
 
+    -- Make sure all the inputs are defined
+    let
+      rst0' = unsafePerformIO . makeDefinedRandom <$> unsafeToActiveHigh rst0
+      timerInterrupt' = unsafePerformIO . makeDefinedRandom <$> timerInterrupt
+      externalInterrupt' = unsafePerformIO . makeDefinedRandom <$> externalInterrupt
+      softwareInterrupt' = unsafePerformIO . makeDefinedRandom <$> softwareInterrupt
+      iBus_ACK' = unsafePerformIO . makeDefinedRandom <$> iBus_ACK
+      iBus_DAT_MISO' = unsafePerformIO . makeDefinedRandom <$> iBus_DAT_MISO
+      iBus_ERR' = unsafePerformIO . makeDefinedRandom <$> iBus_ERR
+      dBus_ACK' = unsafePerformIO . makeDefinedRandom <$> dBus_ACK
+      dBus_DAT_MISO' = unsafePerformIO . makeDefinedRandom <$> dBus_DAT_MISO
+      dBus_ERR' = unsafePerformIO . makeDefinedRandom <$> dBus_ERR
+      jtag_TCK' = unsafePerformIO . makeDefinedRandom <$> jtag_TCK
+      jtag_TMS' = unsafePerformIO . makeDefinedRandom <$> jtag_TMS
+      jtag_TDI' = unsafePerformIO . makeDefinedRandom <$> jtag_TDI
+
     let
       nonCombInput = NON_COMB_INPUT
-        <$> (boolToBit <$> unsafeToActiveHigh rst0)
-        <*> timerInterrupt
-        <*> externalInterrupt
-        <*> softwareInterrupt
+        <$> (boolToBit <$> rst0')
+        <*> timerInterrupt'
+        <*> externalInterrupt'
+        <*> softwareInterrupt'
 
       combInput = COMB_INPUT
-        <$> (boolToBit <$> iBus_ACK)
-        <*> (unpack    <$> iBus_DAT_MISO)
-        <*> (boolToBit <$> iBus_ERR)
-        <*> (boolToBit <$> dBus_ACK)
-        <*> (unpack    <$> dBus_DAT_MISO)
-        <*> (boolToBit <$> dBus_ERR)
-        <*> jtag_TCK
-        <*> jtag_TMS
-        <*> jtag_TDI
+        <$> (boolToBit <$> iBus_ACK')
+        <*> (unpack    <$> iBus_DAT_MISO')
+        <*> (boolToBit <$> iBus_ERR')
+        <*> (boolToBit <$> dBus_ACK')
+        <*> (unpack    <$> dBus_DAT_MISO')
+        <*> (boolToBit <$> dBus_ERR')
+        <*> jtag_TCK'
+        <*> jtag_TMS'
+        <*> jtag_TDI'
 
       simInitThenCycles ::
         Signal dom NON_COMB_INPUT ->
diff --git a/clash-vexriscv/src/VexRiscv/Random.hs b/clash-vexriscv/src/VexRiscv/Random.hs
new file mode 100644
index 0000000..59ed029
--- /dev/null
+++ b/clash-vexriscv/src/VexRiscv/Random.hs
@@ -0,0 +1,49 @@
+-- SPDX-FileCopyrightText: 2024 Google LLC
+--
+-- SPDX-License-Identifier: Apache-2.0
+module VexRiscv.Random where
+
+import Clash.Prelude
+import Numeric.Natural
+import Clash.Sized.Internal.BitVector
+import System.Random
+
+class DefinedRandom a where
+  makeDefinedRandom :: a -> IO a
+
+instance DefinedRandom Bool where
+  makeDefinedRandom b
+    | hasUndefined b = randomIO
+    | otherwise = pure b
+
+instance DefinedRandom Bit where
+  makeDefinedRandom b@(Bit 0 _) = pure b
+  makeDefinedRandom _ = do
+    d <- randomRIO (0, 1)
+    pure (Bit 1 d)
+
+instance KnownNat n => DefinedRandom (BitVector n) where
+  makeDefinedRandom :: KnownNat n => BitVector n -> IO (BitVector n)
+  makeDefinedRandom (BV mask dat) = do
+    let
+      (BV _ maxVal) = (maxBound :: BitVector n)
+      (BV maxMask _) = deepErrorX "" :: BitVector n
+    randomInt <- genNatural (0, fromIntegral maxVal)
+
+    pure $ BV 0 ((dat .&. (maxMask `xor` mask)) .|. (randomInt .&. mask))
+
+genNatural :: (Natural, Natural) -> IO Natural
+genNatural (lo, hi)
+  | lo > hi = error "genNatural: lower bound > upper bound"
+  | otherwise = (lo +) <$> go (hi - lo)
+ where
+  intMax = fromIntegral (maxBound :: Int)
+  intBits = finiteBitSize (0 :: Int)
+
+  go :: Natural -> IO Natural
+  go h
+    | h <= intMax =  fmap fromIntegral (randomRIO (0, fromIntegral h) :: IO Int)
+    | otherwise = do
+    x <- fmap fromIntegral (randomRIO (0, maxBound) :: IO Int)
+    y <- go (shiftR h intBits)
+    pure (x + shiftL y intBits)
diff --git a/clash-vexriscv/tests/unittests/Tests/VexRiscv/Random.hs b/clash-vexriscv/tests/unittests/Tests/VexRiscv/Random.hs
new file mode 100644
index 0000000..3127871
--- /dev/null
+++ b/clash-vexriscv/tests/unittests/Tests/VexRiscv/Random.hs
@@ -0,0 +1,27 @@
+-- SPDX-FileCopyrightText: 2024 Google LLC
+--
+-- SPDX-License-Identifier: Apache-2.0
+module Tests.VexRiscv.Random where
+
+import Clash.Prelude
+
+import Hedgehog
+import Test.Tasty
+import Test.Tasty.Hedgehog
+
+import VexRiscv.Random
+
+import qualified Hedgehog.Gen as Gen
+import qualified Hedgehog.Range as Range
+
+tests :: TestTree
+tests = testGroup "VexRiscv.Random"
+  [ testProperty "genNatural" prop_genNatural
+  ]
+
+prop_genNatural :: Property
+prop_genNatural = property $ do
+  lo <- forAll $ Gen.integral (Range.linear 0 (shiftL 1 1024))
+  hi <- forAll $ Gen.integral (Range.linear lo (shiftL 1 1024))
+  n <- evalIO $ genNatural (lo, hi)
+  assert ((n >= lo) && (n <= hi))
diff --git a/clash-vexriscv/tests/unittests/main.hs b/clash-vexriscv/tests/unittests/main.hs
index 4793f75..03a50be 100644
--- a/clash-vexriscv/tests/unittests/main.hs
+++ b/clash-vexriscv/tests/unittests/main.hs
@@ -10,10 +10,12 @@ import Test.Tasty
 import Test.Tasty.Hedgehog
 
 import qualified Tests.VexRiscv.ClockTicks
+import qualified Tests.VexRiscv.Random
 
 tests :: TestTree
 tests = testGroup "Tests"
   [ Tests.VexRiscv.ClockTicks.tests
+  , Tests.VexRiscv.Random.tests
   ]
 
 setDefaultHedgehogTestLimit :: HedgehogTestLimit -> HedgehogTestLimit