diff --git a/src/operators.js b/src/operators.js
index e233416e2..ba93de411 100644
--- a/src/operators.js
+++ b/src/operators.js
@@ -5,7 +5,11 @@ var operators = {
     '<': (l, r) => l < r,
     '>=': (l, r) => l >= r,
     '<=': (l, r) => l <= r,
-    'contains': (l, r) => l.indexOf(r) > -1,
+    'contains': (l, r) => {
+        if (!l) return false;
+        if (typeof l.indexOf !== 'function') return false;
+        return l.indexOf(r) > -1;
+    },
     'and': (l, r) => l && r,
     'or': (l, r) => l || r
 };
diff --git a/test/syntax.js b/test/syntax.js
index b57a85b6b..1b47aa646 100644
--- a/test/syntax.js
+++ b/test/syntax.js
@@ -13,7 +13,9 @@ describe('expression', function() {
         scope = Scope.factory({
             one: 1,
             two: 2,
-            x: 'XXX'
+            x: 'XXX',
+            y: undefined,
+            z: null
         });
     });
 
@@ -22,12 +24,6 @@ describe('expression', function() {
         expect(evalValue('"foo"')).to.equal("foo");
     });
 
-    it('should throw on illegal expression', function() {
-        expect(function() {
-            evalExp('1 contains "x"', scope);
-        }).to.throw();
-    });
-
     it('should eval variables', function() {
         expect(evalValue('23', scope)).to.equal(23);
         expect(evalValue('one', scope)).to.equal(1);
@@ -48,6 +44,11 @@ describe('expression', function() {
             expect(evalExp('one<=two', scope)).to.equal(true);
             expect(evalExp('x contains "x"', scope)).to.equal(false);
             expect(evalExp('x contains "X"', scope)).to.equal(true);
+            expect(evalExp('1 contains "x"', scope)).to.equal(false);
+            expect(evalExp('y contains "x"', scope)).to.equal(false);
+            expect(evalExp('z contains "x"', scope)).to.equal(false);
+            expect(evalExp('(1..5) contains 3', scope)).to.equal(true);
+            expect(evalExp('(1..5) contains 6', scope)).to.equal(false);
             expect(evalExp('"<=" == "<="', scope)).to.equal(true);
         });