diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## 1.2.0.0
+
+### Breaking changes
+
+* Remove invariants for `StrictTVar`s.
+
 ## 1.1.0.1
 
 ### Non-breaking changes
diff --git a/src/Control/Concurrent/Class/MonadSTM/Strict/TMVar.hs b/src/Control/Concurrent/Class/MonadSTM/Strict/TMVar.hs
--- a/src/Control/Concurrent/Class/MonadSTM/Strict/TMVar.hs
+++ b/src/Control/Concurrent/Class/MonadSTM/Strict/TMVar.hs
@@ -40,10 +40,6 @@
 type LazyTMVar   m = Lazy.TMVar m
 
 -- | 'TMVar' that keeps its value in WHNF at all times
---
--- Does not support an invariant: if the invariant would not be satisfied,
--- we would not be able to put a value into an empty TMVar, which would lead
--- to very hard to debug bugs where code is blocked indefinitely.
 newtype StrictTMVar m a = StrictTMVar { toLazyTMVar :: LazyTMVar m a }
 
 fromLazyTMVar :: LazyTMVar m a -> StrictTMVar m a
diff --git a/src/Control/Concurrent/Class/MonadSTM/Strict/TVar.hs b/src/Control/Concurrent/Class/MonadSTM/Strict/TVar.hs
--- a/src/Control/Concurrent/Class/MonadSTM/Strict/TVar.hs
+++ b/src/Control/Concurrent/Class/MonadSTM/Strict/TVar.hs
@@ -1,9 +1,6 @@
-{-# LANGUAGE BangPatterns       #-}
-{-# LANGUAGE CPP                #-}
-{-# LANGUAGE ExplicitNamespaces #-}
-{-# LANGUAGE GADTs              #-}
-{-# LANGUAGE NamedFieldPuns     #-}
-{-# LANGUAGE TypeOperators      #-}
+{-# LANGUAGE BangPatterns  #-}
+{-# LANGUAGE TypeFamilies  #-}
+{-# LANGUAGE TypeOperators #-}
 
 -- | This module corresponds to `Control.Concurrent.STM.TVar` in "stm" package
 --
@@ -16,8 +13,6 @@
   , castStrictTVar
   , newTVar
   , newTVarIO
-  , newTVarWithInvariant
-  , newTVarWithInvariantIO
   , readTVar
   , readTVarIO
   , writeTVar
@@ -25,8 +20,6 @@
   , stateTVar
   , swapTVar
   , check
-    -- ** Low-level API
-  , checkInvariant
     -- * MonadLabelSTM
   , labelTVar
   , labelTVarIO
@@ -38,22 +31,11 @@
 import qualified Control.Concurrent.Class.MonadSTM.TVar as Lazy
 import           Control.Monad.Class.MonadSTM hiding (traceTVar, traceTVarIO)
 
-import           GHC.Stack
-
-
-type LazyTVar    m = Lazy.TVar m
+type LazyTVar m = Lazy.TVar m
 
-#if CHECK_TVAR_INVARIANT
-data StrictTVar m a = StrictTVar
-   { invariant :: !(a -> Maybe String)
-     -- ^ Invariant checked whenever updating the 'StrictTVar'.
-   , tvar      :: !(LazyTVar m a)
-   }
-#else
-newtype StrictTVar m a = StrictTVar
-   { tvar      :: LazyTVar m a
-   }
-#endif
+newtype StrictTVar m a = StrictTVar {
+    tvar :: LazyTVar m a
+  }
 
 labelTVar :: MonadLabelledSTM m => StrictTVar m a -> String -> STM m ()
 labelTVar StrictTVar { tvar } = Lazy.labelTVar tvar
@@ -76,8 +58,7 @@
 
 castStrictTVar :: LazyTVar m ~ LazyTVar n
                => StrictTVar m a -> StrictTVar n a
-castStrictTVar v@StrictTVar {tvar} =
-    mkStrictTVar (getInvariant v) tvar
+castStrictTVar StrictTVar {tvar} = StrictTVar {tvar}
 
 -- | Get the underlying @TVar@
 --
@@ -87,39 +68,13 @@
 toLazyTVar StrictTVar { tvar } = tvar
 
 fromLazyTVar :: LazyTVar m a -> StrictTVar m a
-fromLazyTVar tvar =
-#if CHECK_TVAR_INVARIANT
-  StrictTVar { invariant = const Nothing
-             , tvar
-             }
-#else
-  StrictTVar { tvar }
-#endif
+fromLazyTVar = StrictTVar
 
 newTVar :: MonadSTM m => a -> STM m (StrictTVar m a)
-newTVar !a = (\tvar -> mkStrictTVar (const Nothing) tvar)
-         <$> Lazy.newTVar a
+newTVar !a = StrictTVar <$> Lazy.newTVar a
 
 newTVarIO :: MonadSTM m => a -> m (StrictTVar m a)
-newTVarIO = newTVarWithInvariantIO (const Nothing)
-
-newTVarWithInvariant :: (MonadSTM m, HasCallStack)
-                     => (a -> Maybe String) -- ^ Invariant (expect 'Nothing')
-                     -> a
-                     -> STM m (StrictTVar m a)
-newTVarWithInvariant  invariant !a =
-        checkInvariant (invariant a) $
-        (\tvar -> mkStrictTVar invariant tvar)
-    <$> Lazy.newTVar a
-
-newTVarWithInvariantIO :: (MonadSTM m, HasCallStack)
-                       => (a -> Maybe String) -- ^ Invariant (expect 'Nothing')
-                       -> a
-                       -> m (StrictTVar m a)
-newTVarWithInvariantIO  invariant !a =
-        checkInvariant (invariant a) $
-        (\tvar -> mkStrictTVar invariant tvar)
-    <$> Lazy.newTVarIO a
+newTVarIO !a = StrictTVar <$> Lazy.newTVarIO a
 
 readTVar :: MonadSTM m => StrictTVar m a -> STM m a
 readTVar StrictTVar { tvar } = Lazy.readTVar tvar
@@ -127,10 +82,8 @@
 readTVarIO :: MonadSTM m => StrictTVar m a -> m a
 readTVarIO StrictTVar { tvar } = Lazy.readTVarIO tvar
 
-writeTVar :: (MonadSTM m, HasCallStack) => StrictTVar m a -> a -> STM m ()
-writeTVar v !a =
-    checkInvariant (getInvariant v a) $
-    Lazy.writeTVar (tvar v) a
+writeTVar :: MonadSTM m => StrictTVar m a -> a -> STM m ()
+writeTVar v !a = Lazy.writeTVar (tvar v) a
 
 modifyTVar :: MonadSTM m => StrictTVar m a -> (a -> a) -> STM m ()
 modifyTVar v f = readTVar v >>= writeTVar v . f
@@ -147,34 +100,3 @@
     a <- readTVar v
     writeTVar v a'
     return a
-
-
-{-------------------------------------------------------------------------------
-  Dealing with invariants
--------------------------------------------------------------------------------}
-
-getInvariant :: StrictTVar m a -> a -> Maybe String
-mkStrictTVar :: (a -> Maybe String) -> Lazy.TVar m a -> StrictTVar m a
-
--- | Check invariant (if enabled) before continuing
---
--- @checkInvariant mErr x@ is equal to @x@ if @mErr == Nothing@, and throws
--- an error @err@ if @mErr == Just err@.
---
--- This is exported so that other code that wants to conditionally check
--- invariants can reuse the same logic, rather than having to introduce new
--- per-package flags.
-checkInvariant :: HasCallStack => Maybe String -> a -> a
-
-#if CHECK_TVAR_INVARIANT
-getInvariant StrictTVar {invariant} = invariant
-mkStrictTVar invariant  tvar = StrictTVar {invariant, tvar}
-
-checkInvariant Nothing    k = k
-checkInvariant (Just err) _ = error $ "Invariant violation: " ++ err
-#else
-getInvariant _               = \_ -> Nothing
-mkStrictTVar _invariant tvar = StrictTVar {tvar}
-
-checkInvariant _err       k  = k
-#endif
diff --git a/strict-stm.cabal b/strict-stm.cabal
--- a/strict-stm.cabal
+++ b/strict-stm.cabal
@@ -1,6 +1,6 @@
 cabal-version:       3.0
 name:                strict-stm
-version:             1.1.0.1
+version:             1.2.0.0
 synopsis:            Strict STM interface polymorphic over stm implementation.
 description:
   Strict STM interface provided on top of
@@ -23,11 +23,6 @@
   location: https://github.com/input-output-hk/io-sim
   subdir:   strict-stm
 
-flag checktvarinvariant
-  Description: Enable runtime invariant checks on StrictT(M)Var
-  Manual: True
-  Default: False
-
 flag asserts
   description: Enable assertions
   manual:      False
@@ -57,7 +52,7 @@
                        array,
                        stm         >= 2.5 && <2.6,
 
-                       io-classes  >= 1.0 && <1.2
+                       io-classes ^>= 1.2
   ghc-options:         -Wall
                        -Wno-unticked-promoted-constructors
                        -Wcompat
@@ -68,6 +63,3 @@
 
   if flag(asserts)
     ghc-options: -fno-ignore-asserts
-
-  if flag(checktvarinvariant)
-    cpp-options: -DCHECK_TVAR_INVARIANT