diff --git a/src/main/java/com/hubspot/jinjava/el/ext/JinjavaListELResolver.java b/src/main/java/com/hubspot/jinjava/el/ext/JinjavaListELResolver.java
index 975286eba..cea4ac484 100644
--- a/src/main/java/com/hubspot/jinjava/el/ext/JinjavaListELResolver.java
+++ b/src/main/java/com/hubspot/jinjava/el/ext/JinjavaListELResolver.java
@@ -64,10 +64,13 @@ private static boolean isResolvable(Object base) {
    *             if property cannot be coerced to an integer.
    */
   private static int toIndex(Object property) {
-    int index = 0;
+    int index;
     if (property instanceof Number) {
       index = ((Number) property).intValue();
     } else if (property instanceof String) {
+      if (!isNumeric((String) property)) {
+        throw new IllegalArgumentException("Cannot parse list index: " + property);
+      }
       try {
         // ListELResolver uses valueOf, but findbugs complains.
         index = Integer.parseInt((String) property);
@@ -92,4 +95,17 @@ public void setValue(ELContext context, Object base, Object property, Object val
       super.setValue(context, base, property, value);
     } catch (IllegalArgumentException ignored) {}
   }
+
+  public static boolean isNumeric(final CharSequence cs) {
+    if (cs == null || cs.length() == 0) {
+      return false;
+    }
+    final int sz = cs.length();
+    for (int i = 0; i < sz; i++) {
+      if (!Character.isDigit(cs.charAt(i)) && cs.charAt(i) != '-') {
+        return false;
+      }
+    }
+    return true;
+  }
 }
diff --git a/src/test/java/com/hubspot/jinjava/el/ExpressionResolverPerformanceTest.java b/src/test/java/com/hubspot/jinjava/el/ExpressionResolverPerformanceTest.java
index d9049ada3..8338a351c 100644
--- a/src/test/java/com/hubspot/jinjava/el/ExpressionResolverPerformanceTest.java
+++ b/src/test/java/com/hubspot/jinjava/el/ExpressionResolverPerformanceTest.java
@@ -40,9 +40,7 @@ public void startTimer() {
     }
 
     public void stopTimer() {
-      System.out.println(
-        String.format("%d msec", System.currentTimeMillis() - startTime)
-      );
+      System.out.printf("%d msec%n", System.currentTimeMillis() - startTime);
     }
 
     public void testMapResolver(int iterations) {