diff --git a/Makefile b/Makefile
index 2dbee3cd7..1dc58f5cb 100644
--- a/Makefile
+++ b/Makefile
@@ -92,7 +92,8 @@ STACK ?= stack
 #STACK=stack --with-hpack=hpack-0.20
 
 # --threads=16 sometimes gives "commitAndReleaseBuffer: resource vanished (Broken pipe)" but seems harmless
-SHELLTESTOPTS=--execdir --threads=16 --exclude=/_  --hide-successes 
+# More threads and/or testing unoptimised builds requires a larger timeout.
+SHELLTESTOPTS=--execdir --threads=16 --timeout=10 --exclude=/_  --hide-successes 
 
 # make sure shelltest is a released version of shelltestrunner
 # run shell tests using the executable specified in tests