diff --git a/src/main/java/jvc/service/Toolbox.java b/src/main/java/jvc/service/Toolbox.java
index e8eb0b8..ece7b70 100644
--- a/src/main/java/jvc/service/Toolbox.java
+++ b/src/main/java/jvc/service/Toolbox.java
@@ -63,7 +63,12 @@ public class Toolbox {
     }
 
     public static BigDecimal evalBig(String val, Map<String, Object> ctx, JsEngine js) {
-        return big(eval(val, ctx, js));
+        final String resolved = eval(val, ctx, js);
+        try {
+            return empty(resolved) ? die("evalBig: error resolving value: '"+val+"'") : big(resolved);
+        } catch (NumberFormatException nfe) {
+            return evalBig(resolved, ctx, js);
+        }
     }
 
     public static BigDecimal evalBig(String val, Map<String, Object> ctx, JsEngine js, BigDecimal defaultValue) {
diff --git a/src/test/resources/tests/test_scale.jvc b/src/test/resources/tests/test_scale.jvc
index aee0e0c..20fd852 100644
--- a/src/test/resources/tests/test_scale.jvc
+++ b/src/test/resources/tests/test_scale.jvc
@@ -1,7 +1,7 @@
 {
   "vars": [
     {"name": "out_width", "value": "1024"},
-    {"name": "out_height", "value": "768"}
+    {"name": "out_height", "value": "source.height / 2"}
   ],
   "assets": [
     // wildcard matches multiple files, vid1_splits becomes a "list" asset. resolution is 320x240
@@ -17,8 +17,8 @@
       "height": "out_height",          // height of scaled asset. if omitted and width is present, height will be proportional
                                        // here we reference the `out_height` variable defined above in the `vars` array
       "validate": [{
-        "comment": "expect output resolution of 1024x768",
-        "test": "output.width === 1024 && output.height === 768"
+        "comment": "expect output resolution of 1024x(source.height/2)",
+        "test": "output.width === 1024 && output.height === source.height/2"
       }]
     },
     {