Error map
NullReferenceException System.NullReferenceException: Object reference not set to an instance of an object
.NET · layer 4, Application & API
Centre: this error. First ring: errors it is linked with (9). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.
Directly related (9)
TypeError: Cannot read properties of undefined (reading 'x')
You tried to access a property on a value that is `undefined` (or `null`) — usually data that hasn’t loaded yet or a wrong proper…
java.lang.NullPointerException
Code called a method or accessed a field on a reference that is `null`.
Fatal error: Uncaught Error: Call to a member function x() on null
Code called a method on a variable that is null instead of an object.
InvalidOperationException: Unable to resolve service for type 'X' while attempting to activate 'Y'
ASP.NET Core’s dependency injection container can’t build Y because one of its constructor dependencies (X) was never registered.
Fatal error: Unexpectedly found nil while unwrapping an Optional value
Swift force-unwrapped (`!`) an optional that was nil, crashing the app.
Blazor: An unhandled error has occurred. Reload 🗙
A Blazor component threw an exception; Blazor Server tears down the circuit (or WebAssembly logs to the console) and shows the re…
InvalidOperationException: Sequence contains no elements / more than one element
LINQ `First()`, `Single()`, `Last()` or `Max()` ran on an empty sequence (or `Single()` found several items).
Azure Functions: Value cannot be null. (Parameter 'connectionString') / Missing value for AzureWebJobsStorage
A Functions app setting or binding connection string is missing — most often AzureWebJobsStorage locally (local.settings.json) or…
warning CS8618: Non-nullable property 'X' must contain a non-null value when exiting constructor. Consider declaring the property as nullable
With nullable reference types enabled, a non-nullable reference property isn’t initialised by the constructor.
Two steps away (27)
TypeError: x is not a function
You called something as a function, but its value is not a function (it’s undefined, a string, an object…).
ReferenceError: x is not defined
Code refers to a variable or function name that doesn’t exist in the current scope.
Error: Objects are not valid as a React child
You tried to render a plain object (or a Promise/Date) directly in JSX; React can only render strings, numbers, elements and arra…
TypeError: 'NoneType' object is not subscriptable
You used `[ ]` on a value that is `None` — typically a function that returned nothing.
java.lang.ClassNotFoundException / NoClassDefFoundError
The JVM couldn’t find a class on the classpath at runtime.
java.lang.OutOfMemoryError: Java heap space
The JVM ran out of heap memory and garbage collection couldn’t free enough.
PHP Fatal error: Allowed memory size of X bytes exhausted
The script tried to use more memory than PHP’s `memory_limit` allows.
PHP Fatal error: Maximum execution time of 30 seconds exceeded
The script ran longer than `max_execution_time` and PHP stopped it.
PHP Fatal error: Uncaught Error: Call to undefined function
PHP can’t find the function you called — usually a missing extension, missing include, or namespace issue.
HTTP Error 500.30 - ASP.NET Core app failed to start
IIS/Azure’s ASP.NET Core Module launched your app but it crashed during startup.
JsonException: A possible object cycle was detected
System.Text.Json found a reference loop (e.g. Order → Customer → Orders → …) while serialising, usually from EF Core navigation p…
UnsatisfiedDependencyException / NoSuchBeanDefinitionException: No qualifying bean of type 'X' available
Spring couldn’t find (or choose) a bean to inject.
java.util.NoSuchElementException: No value present
Code asked for an element that doesn’t exist — Optional.get() on empty, iterator.next() past the end, Scanner with no input.
FATAL: the database system is in recovery mode
PostgreSQL crashed (often OOM-killed) and is replaying WAL to recover.
Illegal instruction (core dumped) (SIGILL)
The program used a CPU instruction your processor doesn’t support — typically AVX/AVX2 in prebuilt binaries (TensorFlow, MongoDB…
CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views
Background code updated a View directly; UI changes must happen on the main thread.
WindowManager$BadTokenException: Unable to add window -- token null is not valid; is your activity running?
A dialog or popup was shown with a context whose Activity is finishing/destroyed, or with the Application context.
ObjectDisposedException: Cannot access a disposed context instance
Code used a DbContext after its scope ended — usually async void, fire-and-forget tasks, or capturing a scoped context in a singl…
Subquery returns more than 1 row (MySQL 1242) / more than one row returned by a subquery used as an expression
A subquery used where a single value is expected (`= (SELECT …)`) returned several rows.
EF Core: The LINQ expression 'x' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly
The query uses C# code EF Core can’t convert to SQL — custom methods, unsupported string/date functions, or complex projections.
ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection
Code accessed a List/collection position that doesn’t exist (IndexOutOfRangeException for arrays).
HTTP 413 Content Too Large (Request Entity Too Large)
The request body is bigger than the server is willing to accept — usually a file upload over the configured limit.
OAuth Error: redirect_uri_mismatch
The redirect URI in your OAuth request doesn’t exactly match one registered for the client in the provider’s console.
WordPress: Error Establishing a Database Connection
WordPress can’t connect to its MySQL/MariaDB database with the credentials in wp-config.php, or the database server is down.
error CS0246: The type or namespace name 'X' could not be found (are you missing a using directive or an assembly reference?)
The compiler doesn’t know the type — missing using, missing NuGet package/project reference, or a target framework mismatch.
error CS0103: The name 'x' does not exist in the current context
The identifier isn’t declared in scope — typo, variable declared in another block, missing using static, or a Razor/partial class…
error CS1061: 'X' does not contain a definition for 'Y' and no accessible extension method 'Y' accepting a first argument of type 'X' could be found
The member you call doesn’t exist on that type — typo, wrong type, missing using for an extension method, or an API removed in a…
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