diff --git a/ansi-terminal/unix/System/Console/ANSI/Internal.hs b/ansi-terminal/unix/System/Console/ANSI/Internal.hs
index 4dee6ba..63faa50 100644
--- a/ansi-terminal/unix/System/Console/ANSI/Internal.hs
+++ b/ansi-terminal/unix/System/Console/ANSI/Internal.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE Safe #-}
+{-# LANGUAGE Trustworthy #-}
 
 module System.Console.ANSI.Internal
   ( getReportedCursorPosition
@@ -11,6 +11,7 @@ import Data.List ( uncons )
 import Data.Maybe ( fromMaybe, mapMaybe )
 import System.Environment ( lookupEnv )
 import System.IO ( Handle, hIsTerminalDevice, hIsWritable )
+import System.IO.Unsafe ( unsafePerformIO )
 import System.Timeout ( timeout )
 
 import System.Console.ANSI.Types ( ConsoleLayer (..) )
@@ -72,8 +73,16 @@ hSupportsANSI :: Handle -> IO Bool
 -- (https://github.com/hspec/hspec/commit/d932f03317e0e2bd08c85b23903fb8616ae642bd)
 hSupportsANSI h = (&&) <$> hIsWritable h <*> hSupportsANSI'
  where
-  hSupportsANSI' = (&&) <$> hIsTerminalDevice h <*> isNotDumb
-  isNotDumb = (/= Just "dumb") <$> lookupEnv "TERM"
+  hSupportsANSI' = (&& isNotDumb) <$> hIsTerminalDevice h
 
 hNowSupportsANSI :: Handle -> IO Bool
 hNowSupportsANSI = hSupportsANSI
+
+-- | This function assumes that once it is first established whether or not the
+-- TERM environment variable exits with contents dumb, that will not change.
+-- This approach is taken because the use of C function setenv() in one thread
+-- can cause other threads calling C function getenv() to crash. On Unix-like
+-- operating systems, System.Environment.lookupEnv is implemented using C
+-- function getenv(). 
+isNotDumb :: Bool
+isNotDumb = unsafePerformIO (lookupEnv "TERM") /= Just "dumb"