diff --git a/org/w3c/css/values/CssCalc.java b/org/w3c/css/values/CssCalc.java
index ae203c3e5..e2b5246bb 100644
--- a/org/w3c/css/values/CssCalc.java
+++ b/org/w3c/css/values/CssCalc.java
@@ -410,6 +410,14 @@ public CssLength getLength() throws InvalidParamException {
                 return val2.getLength();
             }
         }
+        if (computed_type == CssTypes.CSS_NUMBER) {
+            if (val1.getType() == CssTypes.CSS_NUMBER) {
+                return val1.getLength();
+            }
+            if (val2.getType() == CssTypes.CSS_NUMBER) {
+                return val2.getLength();
+            }
+        }
         throw new ClassCastException("unknown");
     }