fixed it

Author: NullVoxPopuli

src/JsonApiDotNetCore/Extensions/IQueryableExtensions.cs

@@ -127,9 +127,14 @@ public static IQueryable<TSource> Filter<TSource>(this IQueryable<TSource> sourc
                     return source.Where(lambdaIn);
                 }
                 else
-                {   // convert the incoming value to the target value type
+                {   
+                    var isNullabe = IsNullable(property.PropertyType);
+                    var propertyValue = filterQuery.PropertyValue;
+                    var value = isNullabe && propertyValue == "" ? null : propertyValue;
+
+                    // convert the incoming value to the target value type
                     // "1" -> 1
-                    var convertedValue = TypeHelper.ConvertType(filterQuery.PropertyValue, property.PropertyType);
+                    var convertedValue = TypeHelper.ConvertType(value, property.PropertyType);
                     // {model}
                     var parameter = Expression.Parameter(concreteType, "model");
                     // {model.Id}
@@ -204,6 +209,9 @@ public static IQueryable<TSource> Filter<TSource>(this IQueryable<TSource> sourc
             }
         }
 
+        private static bool IsNullable(Type type) => type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>);
+
+
         private static Expression GetFilterExpressionLambda(Expression left, Expression right, FilterOperations operation)
         {
             Expression body;

test/JsonApiDotNetCoreExampleTests/Acceptance/TodoItemsControllerTests.cs

@@ -100,7 +100,7 @@ public async Task Can_Filter_TodoItems_Using_NotEqual_Operator()
             _context.SaveChanges();
 
             var httpMethod = new HttpMethod("GET");
-            var route = $"/api/v1/todo-items?filter[updated-date]=ne:null";
+            var route = $"/api/v1/todo-items?filter[updated-date]=ne:";
             var request = new HttpRequestMessage(httpMethod, route);
 
             // Act