diff --git a/src/Cryptol/TypeCheck/Solver/Numeric/Interval.hs b/src/Cryptol/TypeCheck/Solver/Numeric/Interval.hs
index 5ba178ea..fe8195c0 100644
--- a/src/Cryptol/TypeCheck/Solver/Numeric/Interval.hs
+++ b/src/Cryptol/TypeCheck/Solver/Numeric/Interval.hs
@@ -1,3 +1,5 @@
+{-# LANGUAGE PatternGuards #-}
+
 -- | An interval interpretation of types.
 module Cryptol.TypeCheck.Solver.Numeric.Interval where