diff --git a/src/ast.js b/src/ast.js
index 6e570af1..57842cef 100644
--- a/src/ast.js
+++ b/src/ast.js
@@ -459,15 +459,15 @@ export class Binary extends Expression {
     }
 
     // Null check for the operations.
-    if (left === null || right === null) {
+    if (left === null || right === null || left === undefined || right === undefined) {
       switch (this.operation) { // eslint-disable-line
       case '+':
-        if (left !== null) return left;
-        if (right !== null) return right;
+        if (left !== null && left !== undefined) return left;
+        if (right !== null && right !== undefined) return right;
         return 0;
       case '-':
-        if (left !== null) return left;
-        if (right !== null) return 0 - right;
+        if (left !== null && left !== undefined) return left;
+        if (right !== null && right !== undefined) return 0 - right;
         return 0;
       }
 
diff --git a/test/ast/binary.spec.js b/test/ast/binary.spec.js
new file mode 100644
index 00000000..311cc334
--- /dev/null
+++ b/test/ast/binary.spec.js
@@ -0,0 +1,48 @@
+import {Binary, LiteralString, LiteralPrimitive} from '../../src/ast';
+import {createScopeForTest} from '../../src/scope';
+
+describe('Binary', () => {
+  it('concats strings', () => {
+    let expression = new Binary('+', new LiteralString('a'), new LiteralString('b'));
+    let scope = createScopeForTest({});
+    expect(expression.evaluate(scope, null)).toBe('ab');
+
+    expression = new Binary('+', new LiteralString('a'), new LiteralPrimitive(null));
+    scope = createScopeForTest({});
+    expect(expression.evaluate(scope, null)).toBe('a');
+
+    expression = new Binary('+', new LiteralPrimitive(null), new LiteralString('b'));
+    scope = createScopeForTest({});
+    expect(expression.evaluate(scope, null)).toBe('b');
+
+    expression = new Binary('+', new LiteralString('a'), new LiteralPrimitive(undefined));
+    scope = createScopeForTest({});
+    expect(expression.evaluate(scope, null)).toBe('a');
+
+    expression = new Binary('+', new LiteralPrimitive(undefined), new LiteralString('b'));
+    scope = createScopeForTest({});
+    expect(expression.evaluate(scope, null)).toBe('b');
+  });
+
+  it('adds numbers', () => {
+    let expression = new Binary('+', new LiteralPrimitive(1), new LiteralPrimitive(2));
+    let scope = createScopeForTest({});
+    expect(expression.evaluate(scope, null)).toBe(3);
+
+    expression = new Binary('+', new LiteralPrimitive(1), new LiteralPrimitive(null));
+    scope = createScopeForTest({});
+    expect(expression.evaluate(scope, null)).toBe(1);
+
+    expression = new Binary('+', new LiteralPrimitive(null), new LiteralPrimitive(2));
+    scope = createScopeForTest({});
+    expect(expression.evaluate(scope, null)).toBe(2);
+
+    expression = new Binary('+', new LiteralPrimitive(1), new LiteralPrimitive(undefined));
+    scope = createScopeForTest({});
+    expect(expression.evaluate(scope, null)).toBe(1);
+
+    expression = new Binary('+', new LiteralPrimitive(undefined), new LiteralPrimitive(2));
+    scope = createScopeForTest({});
+    expect(expression.evaluate(scope, null)).toBe(2);
+  });
+});