diff --git a/Data/Boolean/SatSolver.hs b/Data/Boolean/SatSolver.hs
--- a/Data/Boolean/SatSolver.hs
+++ b/Data/Boolean/SatSolver.hs
@@ -103,7 +103,7 @@
 -- and only if that fails.
 -- 
 isSolvable :: SatSolver -> Bool
-isSolvable = not . null . solve
+isSolvable = not . (null :: [SatSolver] -> Bool) . solve
 
 
 -- private helper functions
diff --git a/incremental-sat-solver.cabal b/incremental-sat-solver.cabal
--- a/incremental-sat-solver.cabal
+++ b/incremental-sat-solver.cabal
@@ -1,5 +1,5 @@
 Name:          incremental-sat-solver
-Version:       0.1.7
+Version:       0.1.8
 Cabal-Version: >= 1.6
 Synopsis:      Simple, Incremental SAT Solving as a Library
 Description:   This Haskell library provides an implementation of the
@@ -22,7 +22,7 @@
 Extra-Source-Files: README
 
 Library
-  Build-Depends:    base, containers, mtl
+  Build-Depends:    base < 5, containers, mtl
   Exposed-Modules:  Data.Boolean.SatSolver
   Other-Modules:    Data.Boolean
   Ghc-Options:      -Wall