the equals operator is '==', not '='

json-path-assert/src/test/java/com/jayway/jsonassert/JsonAssertTest.java

@@ -127,7 +127,7 @@ public class JsonAssertTest {
 
     @Test
     public void an_empty_collection() throws Exception {
-        with(JSON).assertThat("$.store.book[?(@.category = 'x')]", emptyCollection());
+        with(JSON).assertThat("$.store.book[?(@.category == 'x')]", emptyCollection());
     }
 
     @Test