diff --git a/core/variant/dictionary.cpp b/core/variant/dictionary.cpp
index f2522a4545cd..025f82a393e3 100644
--- a/core/variant/dictionary.cpp
+++ b/core/variant/dictionary.cpp
@@ -84,16 +84,25 @@ Variant Dictionary::get_value_at_index(int p_index) const {
 }
 
 Variant &Dictionary::operator[](const Variant &p_key) {
-	if (unlikely(_p->read_only)) {
-		if (likely(_p->variant_map.has(p_key))) {
-			*_p->read_only = _p->variant_map[p_key];
+	Variant key = p_key;
+	if (unlikely(!_p->typed_key.validate(key))) {
+		if (_p->typed_fallback == nullptr) {
+			_p->typed_fallback = memnew(Variant);
+		} else {
+			*_p->typed_fallback = Variant();
+		}
+
+		return *_p->typed_fallback;
+	} else if (unlikely(_p->read_only)) {
+		if (likely(_p->variant_map.has(key))) {
+			*_p->read_only = _p->variant_map[key];
 		} else {
 			*_p->read_only = Variant();
 		}
 
 		return *_p->read_only;
 	} else {
-		return _p->variant_map[p_key];
+		return _p->variant_map[key];
 	}
 }
 
diff --git a/modules/gdscript/tests/scripts/analyzer/errors/typed_dictionary_assign_differently_typed_key.gd b/modules/gdscript/tests/scripts/analyzer/errors/typed_dictionary_assign_differently_typed_key.gd
new file mode 100644
index 000000000000..531bb751b281
--- /dev/null
+++ b/modules/gdscript/tests/scripts/analyzer/errors/typed_dictionary_assign_differently_typed_key.gd
@@ -0,0 +1,7 @@
+func get_key():
+	return "key"
+
+func test():
+	var typed: Dictionary[int, int]
+	typed[get_key()] = 0
+	print('not ok')
diff --git a/modules/gdscript/tests/scripts/analyzer/errors/typed_dictionary_assign_differently_typed_key.out b/modules/gdscript/tests/scripts/analyzer/errors/typed_dictionary_assign_differently_typed_key.out
new file mode 100644
index 000000000000..d04f127a5c59
--- /dev/null
+++ b/modules/gdscript/tests/scripts/analyzer/errors/typed_dictionary_assign_differently_typed_key.out
@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Invalid index type "String" for a base of type "Dictionary[int, int]".
diff --git a/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_key.gd b/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_key.gd
new file mode 100644
index 000000000000..2f0b3bd0ebfa
--- /dev/null
+++ b/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_key.gd
@@ -0,0 +1,7 @@
+func get_key() -> Variant:
+	return "key"
+
+func test():
+	var typed: Dictionary[int, int]
+	typed[get_key()] = 0
+	print('not ok')
diff --git a/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_key.out b/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_key.out
new file mode 100644
index 000000000000..44f5957556f5
--- /dev/null
+++ b/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_key.out
@@ -0,0 +1,5 @@
+GDTEST_RUNTIME_ERROR
+>> ERROR
+>> Method/function failed. Returning: false
+>> Attempted to use a variable of type 'String' into a TypedDictionary.Key of type 'int'.
+not ok
diff --git a/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_value.gd b/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_value.gd
new file mode 100644
index 000000000000..607cd1a0016a
--- /dev/null
+++ b/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_value.gd
@@ -0,0 +1,16 @@
+func get_value() -> Variant:
+	return "value"
+
+func test():
+	var typed: Dictionary[int, int]
+
+	# TODO: This SHOULD fail, but the assignment operator detaches the value.
+	typed[0] = get_value()
+
+	# Won't fail, as the key is valid and present; value argument skips check entirely.
+	typed.get_or_add(0, get_value())
+
+	# Will fail.
+	typed.get_or_add(123, get_value())
+
+	print("not ok")
diff --git a/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_value.out b/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_value.out
new file mode 100644
index 000000000000..4bbfe1810bdd
--- /dev/null
+++ b/modules/gdscript/tests/scripts/runtime/errors/typed_dictionary_assign_differently_typed_value.out
@@ -0,0 +1,4 @@
+GDTEST_RUNTIME_ERROR
+>> ERROR
+>> Condition "!_p->typed_value.validate(value, "add")" is true. Returning: value
+not ok