diff --git a/js/csq.js b/js/csq.js
index 09e5cbe..c138f16 100644
--- a/js/csq.js
+++ b/js/csq.js
@@ -8,15 +8,21 @@ var CSQ = function (info_csq, csq_names, numeric_fields) {
 
     return new Proxy(csq, {
         get: function (target, name) {
-            if (name in target._names) {
-                var result = target._values[target._names[name]];
-                if (target._numeric_fields[name]) {
-                    result = parseFloat(result);
-                }
+            if (!(name in target._names)) {
+                throw "unknown CSQ field: " + name;
+            }
 
-                return result
+            var result = target._values[target._names[name]];
+            if (target._numeric_fields[name]) {
+                if(result.length == 0 || isNaN(result)) {
+                    result = NaN
+                } else {
+                    try {
+                        result = parseFloat(result);
+                    } catch(e) { result = NaN }
+                }
             }
-            throw "unknown CSQ field: " + name;
+            return result
         }
     });
 };
@@ -31,4 +37,4 @@ function CSQs(csq_string, vcf_csq, numeric_fields) {
 
     csqs = csq_string.split(',');
     return csqs.map(function (csq) { return new CSQ(csq, vcf_csq, numeric_fields); });
-}
\ No newline at end of file
+}