diff --git a/jukebox.cabal b/jukebox.cabal
--- a/jukebox.cabal
+++ b/jukebox.cabal
@@ -1,5 +1,5 @@
 Name: jukebox
-Version: 0.2.4
+Version: 0.2.5
 Cabal-version: >= 1.8
 Build-type: Simple
 Author: Nick Smallbone
diff --git a/src/Jukebox/SMTLIB.hs b/src/Jukebox/SMTLIB.hs
--- a/src/Jukebox/SMTLIB.hs
+++ b/src/Jukebox/SMTLIB.hs
@@ -165,7 +165,7 @@
       | otherwise = "|" ++ concatMap escapeSym xs ++ "|"
 
     isOk c
-      | isAlpha c = True
+      | isAlphaNum c = True
       | c `elem` ("~!@$%^&*_-+=<>.?/" :: String) = True
       | otherwise = False