diff --git a/README.md b/README.md
index 154bdebe..a3565912 100644
--- a/README.md
+++ b/README.md
@@ -71,7 +71,8 @@ Build settings are configurable in `tup.config`.
 
 - `CONFIG_DEFAULT_OPTIPNG`: Command line to pass to `optipng`.
 - `CONFIG_DEFAULT_ADVPNG`: Command line to pass to `advpng`.
-- `CONFIG_USE_DEFLOPT`: Path to `DeflOpt.exe`. If using Wine, you have to copy it somewhere outside the Tup tree.
+- `CONFIG_DEFLOPT_PATH`: Path to `DeflOpt.exe`. If using Wine, you have to copy it somewhere outside the Tup tree.
+- `CONFIG_DEFAULT_DEFLOPT`: `true`, `false`, or blank
 
 There are src-specific versions of these settings:
 
diff --git a/Tuprules.lua b/Tuprules.lua
index 9a7cc956..c3a99a89 100644
--- a/Tuprules.lua
+++ b/Tuprules.lua
@@ -15,20 +15,31 @@ function pad(w, h, input, output)
     }
 end
 
+function asboolean(s)
+    if s == nil or s == "false" then
+        return false
+    elseif s == "true" then
+        return true
+    else
+        error("boolean config must be true, false, or empty")
+    end
+end
+
 local DEFAULT_OPTIPNG = getconfig("DEFAULT_OPTIPNG")
 local DEFAULT_ADVPNG = getconfig("DEFAULT_ADVPNG")
-local USE_DEFLOPT = getconfig("USE_DEFLOPT")
+local DEFLOPT_PATH = getconfig("DEFLOPT_PATH")
+local DEFAULT_DEFLOPT = asboolean(getconfig("DEFAULT_DEFLOPT"))
 
 function compresspng(opts)
     local cmds = {}
     local output = opts.output or "%o"
     local optipng = DEFAULT_OPTIPNG
     local advpng = DEFAULT_ADVPNG
-    local deflopt = USE_DEFLOPT
+    local deflopt = DEFAULT_DEFLOPT
     if opts.config then
         optipng = getconfig(opts.config .. "_OPTIPNG") or optipng;
         advpng = getconfig(opts.config .. "_ADVPNG") or advpng;
-        deflopt = getconfig(opts.config .. "_DEFLOPT") or deflopt;
+        deflopt = asboolean(getconfig(opts.config .. "_DEFLOPT")) or deflopt;
     end
 
     if optipng then
@@ -38,9 +49,12 @@ function compresspng(opts)
         cmds += rep{"advpng -q {opts} {output}", opts=advpng, output=output}
     end
     if deflopt then
+        if DEFLOPT_PATH == nil then
+            error("Need to set CONFIG_DEFLOPT_PATH")
+        end
         cmds += rep{"node {root}/tools/deflopt {deflopt} {output}",
                     root=ROOTDIR,
-                    deflopt=deflopt,
+                    deflopt=DEFLOPT_PATH,
                     output=output}
     end